probably the 3u for less noise plus more space. back in the day i bought a rosewill 4u and modded it, got backplates that were only psu and pcie slots all the way across, drilled holes and put standoffs for the motherboard in the front of the unit. little bit of work but fit 6 cards in it with a cheap board and still run it to this day. The other way is to get tiered shelf unit for 60usd, you could put 72 cards on a full rack front or double it with cards along both sides and psu motherboards on it nice and cheap just put some big space fans on it to cool everything with gpu fan profiles adjusted up a little from stock. dont get me wrong i like rackmounts but thats alot of money wasted... imo