I agree its some about cost. Personally, I could do fine with a mid 90s AOD and a few upgrade parts because I don't intend to make 1xxx HP ever. 600 would be about the limit my budget could produce and that's plenty. My home built roller C4 has seen a lot of use up to 500 HP with no issues, so I'm sure I could build up a AOD easily for much less than a GV unit.
Yes, for me, I am starting from ground zero, so the AOD makes sense. I do have a C-4, but it's a '67 core, so you would want to do the input shaft upgrade anyway, just for converter selection, by the time you do that, it makes more sense to start with a $100 aod core.