Originally Posted by Rugger_can
You would have to downgrade the spring.
Get an M4, it's easier to downgrade and as you progress you can add bells and whistles to pimp it up. That and mags and vest's/gear is easier to acquire for the m4.
Though I would suggest getting AVed and waiting until you can check out the deals on the classifieds and the retailers here, it could save you a few bucks.
They both come with M100 downgrade springs, though I'm not sure it would provide enough of an FPS drop in either guns case.
I'm getting a pretty good deal on these 2 as it is, and I don't have to pay/wait for shipping or meet some stranger somewhere and show them my ID
EDIT: Now that I look closer at the upgrades both guns come with, I can't think of any more I would likely do in the near future, and since I'm not really a big fan of most of the external mods(flashlights,big sights,etc) I'm going with the MP5. Hopefully G&G would be willing to sell me replacement parts if i emailed them, since I have yet to stumble across any piston heads etc that say they will work with an MP5.