ACR if you plan to convert calibers.
Solely based on what ATRS has posted on this forum how against they are of anyone modifying their product. If you were too ever need replacement parts in the future that are proprietary to the MV/MH, it is unlikely they will sell them to you with what has been said.
PS, I am going to do it anyways when I find a cheap used one
From a liability perspective I wouldn't expect that they would recommend people start changing things. If something goes wrong the first thing the guy will say is "but Rick told me I could". Seeing how Rick has made it clear that he is not a fan of litigation I wouldn't expect him to say anything other than "don't touch it, send it back to us".
As far as the unwillingness to supply proprietary parts goes I can only recall that being said in regards to the part that helps to position the trigger pack. I may have missed something but afaik that 1 part is the only 1 part he has specifically stated that they will not sell to anyone. Seeing as there is a huge safety risk if someone doesn't reinstall it perfectly I don't blame him for that.
How much support does the ACR get from Bushmaster? I'd say ATRS wins in this department. Even if there is 1 little piece they won't sell us.
As far as modifications is concerned, I have already changed a few little things and will continue to do so if I get the itch to change or add something. That is part of the enjoyment of owning something like this. Or the ACR for that matter.
There is no reason for anyone to think they can't do anything to the MV. It's got a few little proprietary differences but in general it is pretty standard stuff.
I'm currently sourcing a 20" .204 ruger barrel for mine for no other reason than I want something different for zapping yotes. 223 is getting boring to me. Most every gun out there is in 223. I want mine to be different. Changes can be made to this gun easily enough. Standard AR barrel with nothing more needed than changing the location of the indexing pin.
If you have the know how (or the budget to pay someone with the know how) calibers that ATRS won't do can be done. No different than having to do it with the ACR.
I do things to mine with the full understanding that if I eff it up I don't expect ATRS to have to fix it for me.
ACR or MV, if you want to make them into something different there is no difference. You take on the responsibility of doing so.
I think they are both great platforms. Maybe one day I will get an ACR.