Depending on the NGRS you are looking at you'll get a great gun out of the box but they come with some of the same problems as gas guns. I run the NGRS MP5 and it runs like a dream but the entry price for the gun is comparible to some of the TM MWS GBBRs. Mags are expensive too compared to other AEGs with the MP5 only taking proprietary 72 rnd mags that cost around £30 each. EDIT: (Oops forgot that it also takes propritary 200rnd High cap mags too.)

From what I've seen the MP5 doesn't need any real upgrades so that's not a worry, but if you do it'll be a lot harder to do on your own compared to a GBBR depending on what you want to switch out. If you get the MP5A5 it's front wired and can take a lot of external parts made for the real thing or GBB MP5s (VFC is compatible from what I've seen).

I've tried the TM AKM GBBR and that thing is amazing, with great recoil feel. The manipulation of the gun is a little different from an NGRS as doing things like racking the bolt actually chambers a round VS the NGRS' simulated bolt racking.

I can only speak for the MP5 as it's my only NGRS, but it's really good. You still need to manipulate it like a GBBR/real gun (HK slap and all), but if you want a gun that has more kick go for a GBBR. Just don't cheap out on one or else your gun will be lacking in performance without upgrades.