Alright this probably isn't going to be a welcome suggestion due to the G5 experience, but I will say it anyway.
GHK M4.
As I said, the G5 was their low budget "accessible" option which sacrifices quality to lower cost. The internals are zinc and the externals are polymer, but the gun simply kicks too hard for the build quality.
The M4, on the other hand, has steel internals out of the box. Unlike most brands, the externals are not crappy pot metal either, they are a high grade aluminium (though RATech offer higher grade 7074 aluminium receivers too). The internal mechanism is realistic and the kick is great. Accuracy is also brilliant, there are many tests on YouTube. However, if you are spending this much on a gun then the chances are that you will eventually switch the hop up rubber and inner barrel anyway. I believe the GHK M4 is compatible with AEG rubbers and inner barrels.
Performance wise, the MWS and GHK M4 are essentially the same, with the MWS having slightly less recoil. While the GHK is a replica, the MWS was designed more towards being an airsoft gun, hence the internal mechanism they've come up with.
These are basically the 2 best M4 GBBRs on the market and you aren't going to go wrong with either choice.
I really miss my GHK M4
I would normally never recommend WE, ever. However, this SCAR-H GBB I have is ridiculously fun. Although the internals of mine have mainly been switched out to the likes of Angry Gun or RATech, with few things remaining as WE. Could be worth a look, the SCAR H mags are very gas efficient due to their size and all the controls are basically ambidextrous with the exception of the bolt release. Even the charging handle can switch sides which is nice. Besides, you can most definitely build a DMR from a SCAR H.
For reference, the Tippmann's hop is not so good.
OR, if performance is what you are after, buy a TM MK23. The one-handed silent sniper. Or put it in a carbine kit.
Gonna stop typing now because I feel like I have no life writing an essay on gas guns :lol: