I applaud that you are considering this as you begin to compile components. Many simply don't care. It's long been my process that when I build a box of 50 rounds of handgun ammo, everything in that box is going to have the same headstamp. Part of that is simple pride and maybe a smidge of OCD that wants it to look "right" but when it comes to anything in this game, consistency
is where the results lie.
The argument of internal capacity is real, but in most handgun rounds, it a non-issue, given the relatively small size we are talking about. You'd have much more chance of seeing tangible differences related to capacity when handloading rifle ammo.
For my buck, however, what it VERY important at the load bench when making pistol ammo is brass thickness (related to case mouth tension) and using a large load of similar brass allows me the exact same feel with every mouth flare and bullet seat. Truly and without a stitch of doubt, I can tell you for certain that if I run a 5 piecee of Winchester (any cal.) through a flare die or a seat/crimp die and then you sneak a piece of R-P in there on me, I will
feel the difference. And if I can feel it in a press lever, I know
it's constructed "differently" enough that it bothers me.
Sure, I'll load up 200 rounds using R-P brass (in most cals.), but I will adjust the flare settings on my die until I get the feel I desire, and I'll do it with my crimp as well. And priming these different cases is the same animal, you'll get a different feel across different headstamps. In my book, that's NOT consistency, and if I don't feel
like I took the proper steps to make consistent ammo, I won't have the same level of confidence in it.
Now, after all that, I have to say two things:
1) There are folks that shoot different kind of competitions that have used mixed brass for years and years and their opinion may differ from mine and that's fine. I'm not saying they are wrong and if it works for them, it's all good. This is what works for me.
2) Even with that whole explanation -- I'd absolutely
buy mixed lots of used brass. For sure. First off, that's pretty much how it's going to be sold 95% of the time. While I sort brass constantly, I realize that I do more of it than most folks would put up with.
Guys that sell brass and often likely to separate out headstamps for you. So unless you wish to buy new brass ($$$!), you'll be buying mixed lots for the most part... or you won't be buying anything.
Secondly... just simply by picking up little gold mines at different range days, you are going to end up with a supply that includes 15 different head stamps anyway. You may as well start stockpiling different cans of it and using it as such. Some days when I make 9mm, I'm making 400 rounds with PMC stamps. Sometimes it's Blazer. Or Winchester. Or fill-in-the-blank. You are going to end up with a lot of the stuff the longer you keep at this game, so a nice purchase of a mixed lot is just a way to accelerate that process.
All of that is my opinion in handgun brass and loading ONLY!
Rifle loading is a different ballgame entirely.