Hmm, so the full suit has 12/10 armor, and you can add PPP and FFBA. It has 8 capacity and 7 in the helmet. Heavy Milspec gives 18/16, but won't stack with anything else. It has a whopping 20 capacity, and the helmet seems to come with most stuff and can be upgraded with the rest without any listed limitation.
In the armor department, the full suit could get up to 20/18, requiring a body of 8 or 9(I forget which way you round). The milspec heavy is 18/16 when paired with the required helmet, and it only requires 6 body.
For upgrades, the FS/PPP/FFBA lets you take 18 ranks of mods plus 8 capacity in the armor itself and 7 in the helmet. The milspec heavy gives a flat 20, but unlimited for the helmet.
The FS/PPP/FFBA costs 9600, while the milspec heavy costs between 30,000 and 48,500 nuyen(depends on what rating you want in the included commlink).
The full armor suit gives you slightly more armor for a significantly cheaper price(at least 20k cheaper) and much lower availability(plus it's not Forbidden, just Restricted). It gives somewhat less total upgradeability(maybe... on the other hand, mods that don't have a rating just count as 1, which is cheaper than actually using capacity), but requires a higher body attribute.
I would say they both have their uses.