I have two rifles in MPA BA Comp chassis, and two in MDT ACC chassis. Both makes are great. The MPAs came first, then I tried an ACC, really liked it, and got another.
In the 5+ years I've been into precision rifle and PRS-style competition, MDT has repeatedly earned my loyalty. So I always steer people like the OP to them - and if you try a chassis or anything else from MDT and don't like it, the item can be returned for refund within 30 days. Email in a question, and you'll get a response quickly - usually the same day and I'm not surprised when an answer comes back in an hour or two.
This in no way criticizes MPA - my few interactions with the company have been satisfactory. It's just that MDT has gone way beyond satisfactory for me personally, like replacing a magazine over a year after its purchase because it was dang near impossible to cram in the last two rounds with my arthritic hands.
Main physical differences between BA Comp and ACC:
- The ACC fore end is LONG - several inches longer than the BA Comp. This allows sliding a bipod WAY forward (of course, both chassis have machined-in ARCA rails)... I have even had a gamer plate and a bipod mounted at the same time - the BA Comp isn't long enough to do that with my gear, although of course the bipod could be mounted on the BA Comp's included spigot.
- That long ACC fore end can get in the way in some safes - I had to cut a row divider out of mine because the fore end was too tall to fit its opening.
- ACC grip has some fore/aft adjustability, BA Comp grip doesnt.
- Vertical grips are shaped a little differently and thus feel a little different. If OP is a big guy with big hands, the new Elite grip that ships with the ACC system may have extra appeal - it's a meaty thing.
- ACC comb and length of pull adjusting "wheels" are "locked" via thumb screws (or included allen-head screws), while BA Comp "wheels" have click-stops. Pros and cons to each.
- I think - I'm not sure - that the MPA chassis ships with an adjustable magazine latch, while the MDT chassis does not. I got one of the early adjustable MPA latches because I was having feed issues with my BA Comp-mounted Vudoo .22; presto, problem fixed. MDT does have different-length latches if needed.
Again, both companies' offerings are great. And, again, if I need something that MDT makes, I don't even look anyplace else because, time after time, MDT has earned my business and trust.
My $0.02.
EDIT: Actually, I've owned four rifles in MDT chassis, not the two I still have. I had a CZ-455 .22 and Tikka T3X Varmint in LSS chassis before I got into custom rifles. I parted out the T3X and sold the chassis to someone out of state; the -455 in the LSS chassis went to someone local and, last time I saw him at a match, he was really happy with it.