I've long been a staunch advocate of the squat, and I'll always remain one. For most people the squat is the single most important exercise for building size and strength. The dead-lift is also one heck of an exercise, however, and shouldn't be sidelined by the squat. For some people the deadlift may be a more productive exercise on which to focus. Note that it is the bent-leg, not the stiff-legged, deadlift that approaches the squat in terms of overall gains.
We cannot fairly compare the squat and deadlift by considering how many people have gained well from the squat compared to those who have gained well from the dead-lift. The squat wins hands down, primarily because more people have used the squat as the linchpin in their gaining routines than have used the deadlift. Just think how much promotion the squat has been given and how little the deadlift has received.
If, for argument's sake, 100,000 people have given the 20-rep squat program a fair trial, while only 1,000 have given the 20-rep deadlift program a fair trial, there will obviously be a lot of successful squat stories but a relatively small number of successful deadlift stories. If the comparison were between similar numbers of equally serious test cases, the numbers of success stories would be closer, although I believe the squat would still come out on top.
Let's not think of this as a squat vs. deadlift situation but rather a dead-lift-squat teamwork situation, with the understanding that for a minority of people the deadlift may be more productive than the squat in a straight comparison. Some people, due to leverage factors, have greater potential as deadlifters than as squatters. No matter how much they work on the squat, they do better on the deadlift, even when they give less effort, the deadlift remains well ahead of the squat. Others have a more natural propensity for the squat, and still others have lousy leverages for both lifts, although these folks will likely have one that is less weaker than the other.
Tall and very thin neophytes in particular may have a far easier time pulling a weight than squatting it. Focusing on the deadlift will enable them to pack on more initial mass and strength than will the squat. With a large accumulation of mass and more attention to form and flexibility, they will become better squatters and be able to prosper on the squat, though they may always be better deadlifters.
Some people will gain fastest when they squat and deadlift, setting the frequency of training each lift according to their individual recovery ability. Others, however (both beginners and advanced lifters) will find this to be too much and will need to back off from one of the lifts to be able to gain well on the other. I know this from personal experience, and I'm sure my findings are shared by many others.
Let's suppose you're a typical hardgainer wanting the best from both the squat and the deadlift. Squat on Tuesday and perhaps moderately on Fridays, and deadlift on Fridays, doing no more than a handful of other exercises at each workout. If both your squat and deadlift move just about equally in terms of poundage gained, keep at it. You may find that one of the two dries up once you're at or beyond your previous best lifts in both exercises. If this happens, the chances are that it's your naturally stronger lift that is gaining, unless of course you were previously slacking on the weaker lift and you're now just getting the balance right. You may, of course, find that both lifts dry up, not just one of them. Assuming that you've been working equally hard on both lifts, what happens at the end of a cycle can tell you a lot.
For example, suppose you're an advanced trainee and you spent a few weeks getting used to the routine. Then during the past nine weeks you slowly built up to 365 x 20 in the rest/pause deadlift and 275 x 20 in the rest/pause squat, up from previous bests of 350 x 20 and 270 x 20, respectively. Rest/pause means that after each rep you pause for a few breaths before continuing, keeping the bar on your back when squatting and not taking your hands from the bar when you're deadlifting. These lifts show a natural bias toward the deadlift.
In the final stage of this cycle-training absolutely full-bore, suppose your poundage gain for the deadlift has been moving at a rate of five pounds per week recently, but the squat has only been moving 2 1/2 pounds per week. An extra 2 1/2 pounds on the squat (up to 277 1/2) makes you fail to get all 20, and you struggle for a few weeks to build up to all 20; however, as brutally hard as the work undoubtedly is, the deadlift keeps moving at five pounds per week.
By the time you've actually worked to getting all 20 squats out, you're deadlifting 380 x 20. The squat is suffering from the effort (both in the gym and in recovery) that you're pouring into the deadlift. In this case, there's no way you're going to gain on the squat while working so hard on the deadlift, so back off on the squat and focus on peaking on the deadlift. Cut back your squat poundage by 20 percent or more for the same reps, and hold it there while going as far as you can in the deadlift, to say, 400 x 20 for this example. Anything from about 350 for 15 rest/pause reps is great deadlifting for a hardgainer.
On the other hand, if despite the fact that you're taking both lifts equally seriously, your deadlift poundage is the same or a little more than you're lifting for the squat (for the same reps), then you're more naturally suited to the squat and the poundages given above would be adjusted accordingly. In this case the chances are that the deadlift would fall behind the squat in terms of poundage gained at the end of the cycle.
In both cases rather than knocking yourself out on both the squat and the deadlift in the same cycle focus on only one of these exercises and work the other moderately to hold style and some conditioning there, but drop it in the very final weeks of the cycle. This will enable you to make good progress on the other lift as you move into new poundages. By reducing the demands on your body, you'll be able to train the priority lift twice a week rather than just once. Make one your full-bore day and the other a light day. This will help reduce the severity of soreness from the heavy day. For your next cycle reverse the focus.
Set a realistic poundage target for a squat-dominated cycle and then realize it. (Advanced lifters should target a smaller percentage increase than less-experienced lifters.) Next set a target for a deadlift-dominated cycle and give your all to realizing it. Then go back to the squat, etc. This is better than training both lifts with equal priority in the same cycle and getting very little on either.
Neglect neither the squat nor the deadlift, regardless of how well or not so well you may be suited to them. Get the best from both.