I used the 7x5 on mine and it had really good thrust and stayed pretty cool. The 6x4 prop did nothing, so my guess is that the 6x5.5 will be just Ok. I'd definitely go with the 7x5. I used my 60 amp esc for a good machine of safety, but I think 50 amp esc would work too.
I use a 6x4 on my NTM 2200kv and i flies great. I have not tried a 7X5 and might do so. F1Wanabe, is it a big difference? Alex