We're considering the fat twins so we get both a lot of CPU and some disk. GitLab.com is pretty CPU heavy because of ruby and the CI runners that we might transfer in the future. So we wanted the maximum CPU per U.
The 2028u has 2.5" drives. For that I only see 2TB drives on http://www.supermicro.com/support/resources/HDD.cfm for the SS2028U-TR4+. How do you suggest getting to 4TB?
If you do somehow manage to pick the perfect disk sure having everything from a single batch would be the best since that'll ensure you have the longest MTBF. But how sure are you that you'll be picking the perfect batch simply by blind luck?
That said, I bought the same 6TB HGST disk for two years.
But when you're buying 100% of your disk inventory at once there's a serious "all eggs in one basket" risk.
As for CPU density, I still feel like you're going to need more spindles to get the IO you're looking for.