Was about to get a toy- then I did some research...

[JAZZCNC]

The round vs profile rail debate is one that has had me torn the most. Undoubtedly profile are superior in theory, in practice, however, I've been somewhat put off by the many worst case scenarios people have posted about when ordering the Chinese kind (which I strongly suspect is the only kind my budget allows).

This is the problem people only focus on those who have had issues, probably because they shout the loudest, but for everyone that as an issue there are 100's more who haven't and work perfectly fine.
Chinese VFD's and spindles being a classic case, I've lost count of how many sets of VFD/Spindles I've fitted and I've had about 3 failures in 10+yrs and those weren't all due to quality but more user error or wiring issues.
I was probably one of the first in UK to use the Cheap Chinese Linear rails and again I've fitted dozens of sets without any issues, Now I fit Hi-win because the price difference isn't too great, but both come from China so don't be put off. Either of them are far superior to round rail and worth the extra, trust me on this you won't regret buying them.

Second, being a first timer, I was hoping to get slightly more parallel mounting tolerance with the round rails (bc self alignment), and they provide more room for mounting ball screws on the same face (although that could also be achieved by spacers on the profile carriages). I'm curious though, in what way do you find profile to be less of a hassle?

Not quite sure what your meaning by Less hassle.? Less hassle than what.? Compared to using steel it's lot less hassle for obvious reason, try drilling and tapping 150 x M5 holes in steel and you'll understand this quickly.! . .Lol

If you are referring to my comment about using 45x90 because of the slot spacing being less hassle. Then I mean it's less hassle than having to make plates which first bolt to the profile so you can then mount the BK/BF bearings onto. Or having to Drill and tap the profile directly, which don't advise doing because never works out good. Like on this gantry that uses 40mm spaced slots and BK15 bearings with larger hole spacing.

With regards the bk/bf12, I thought bolt spacing was something like 46mm in which case standard t-slot nuts wouldn't work? Or is it merely a case of using good ol' force? .

The holes allow just enough wiggle room so that it works ok.

I long favoured flipping the ball screw and stepper positions, but figured ball screw placement as centrally as possible relative to the gantry rails took precedence over drop plate length.

I find comments you've seen made like these are made mostly by armchair theorists who have probably never built a machine.!
Because in the real world the difference between them is so negligible that you'll never see or notice any difference in how the machine performs. I've built all kinds of machines in all possible configurations and I've yet to see one that stands out to even slightest degree that says that's the best way.

End of the day if it fits your design best to have it raised up a little higher then go with it because you won't notice any difference if struggle on and have it the so-called "Optium" position.
The reason I say move it up is that being shorter it won't vibrate as much, which then goes into the screw, which then goes into the motor etc.
Also because the bracket is actually fastened to the top bearing plate anyway, the so-called optimum center position doesn't apply because the forces are being placed on the top bearing plate, not into the center of the Z-axis rear plate which is considered the Optimum position. So having it short means the forces are moved closer to the actual bearing plate which is what's being moved and makes everything that little bit stiffer.

Again don't stress over this stuff because it's not a game-changer or disastrous if not in the optimum place, you WON'T notice any difference. The little details like being stiffer or better-placed screws, motors, switches, etc make for a better more reliable machine than if a ball-screw isn't quite in the Optimum place.