I think the difference is slight. I was about to get the -7s...but then I realized that I'm me and not a normal human, and that I was going to push these turbos to the absolute limit, so I went -5s.
Given a stock ECU, I would definitely NOT touch the -5s. I knew a guy who did, thinking he'd run them at 1 bar until he could afford to upgrade the ECU and fuel system and god knows what else (I've lost count of mods needed to support over 600bhp!), and then blew out his pistons within three months.
Get the -7s (I voted -5s but in your situation its very risky), keep the boost down until you get the FCon and some huge injectors, and then when you remap, get that high boost kick - there's plenty of it at 1.5bar for either turbo. I'm just a madman, that's why I need more
