I'm sure others with more experience can put this better, but here's what I'm noticing. I just got a squat rack last week, so I can finally compare heavy squats with the bar, with weighted vest up to 40 pounds, with dumbbells, and a combination of vest/bar or vest/dumbbells.
* dumbbells were/are great for narrow leg squats, though they put noticably more load on my arms and elbows (and I often need to use VersaGrips to hold the weight). I did notice some leg banging because those adjustables are pretty bit! And, I also have to watch my knees to keep them pointed correctly. Dumbbells were okay for plie squats up to a point, but even with VersaGripps I can't hold enough weight in my hand to push these. 50-55 pounds is about my current limit in hand, and even then, it has to be for only a few reps (maybe 8-9). Dumbbells feel safer, in a way, because I could drop them if they get too heavy. Also, I can change my wrist position for comfort, and I can be more confident that both arms/sides are working evenly. I plan to continue using dumbbells sometimes for squats, because I think it is helping my grip strength improve more than having the bar always on my shoulders would.
* wide legged squats or plie squats are much more comfortable with the bar! I really like the different leg position options with a heavy bar versus dumbbells. I'm still a novice with the squat racks, though. It is taking me three-four workouts to get my balance and bar skills to where I'm comfortable going as heavy with the bar as with dumbbells - learning to rack the bar, making sure I'm not going to hit something with the ends, learning to stand perfectly evenly - lunges will probably take me a fifth workout to get really comfortable with that, since I have to step away from the rack to do them in my small room. My sense is I will improve my strength faster for squats with the bar once I master the squat rack because it does not stress my arms and somehow seems to help me get into better form.
* weighted vest seems 'easiest' to me - it is nicely centered on my body, there is no strain on the arms except to take
it on/off. However, my heaviest weighted vest is 40 pounds so it's an add-on. Can't use them for deadlifts - at least, that's the advice I've gotten. And, there's not the same balance challenge with squats or lunges - I get a feeling that the bar on shoulders is challenging my core, which is good. A person is 'tippier' with a weighted bar on shoulders than with a vest, or dumbbells held the side.
So far the squat combination I like best is the barbell on a rack plus weighted vest. The weighted vest helps me remain confident in being 'even', and the bar is a more comfortable way to hold weight (and gives me more possibilities with foot position). The squat rack lets me load more weight in the bar, and so I can easily go heavier. For what it's worth, I squatted at 144 pounds on the weekend (bar 104, vest 40), doing Meso 3 but with ten reps not 8, since I don't want to challenge my legs to the max until I'm confident in my squat rack technique.
For deadlifts, I am with you, I really love doing them with the bar!!! However, there are a few deadlift variants with leg positions where that doesn't work as well, and I'll use dumbbells for those. Also, from time to time, I'll do dumbbells just to be sure I'm using my body evenly, and to mix things up.
