I have a G&G TopTech M4 and I've had the CYMA CM041 MP5J in the past.
G&G M4- pros
G&G, so nice internals
Decent externals
As it's an M4 there's a lot of aftermarket parts out there for it
There are a LOT of different designs of M4 in G&G's range, from machine pistol size M4s to longer than full length rifles
G&G have better quality control, however the occasional lemon slips past them
M4 mags are very easy to source
If you stick with hicap magazines you get a massive capacity of over 500 rounds (if you can bear with the rattling)
All M4s with a removable carry handle have a top RIS rail for optics as standard. Plenty of G&G M4s come with RIS hand guards as well
ROF tends to be fairly good for a general purpose AEG
Cons:
Externals aren't the nicest on G&G, G&P and VFC are better in that regard
M4s can be a bit picky about magazine compatibility- certain brands work in certain guns, others don't, if you try to be safe and buy G&G mags but you can't find any it, can be a hassle buying mags trying them out and sending them back if they don't feed/fit properly
M4s are the most popular airsoft guns, meaning they are the most common to see at normal skirmishes. That being said some of G&G's M4 range look very alternative to the standard M4, and that massive amount of aftermarket parts I mentioned before will come in helpful if you want to make your M4 more original
CYMA MP5 pros:
Very powerful out of the box
CYMA are second only to TM when it comes to their hop units and rubbers, they're amazing for a cheap gun
As it's an MP5, they tend to be more compact than the standard length M4s- completely telescopic stock on the J model helps with this
MP5s are marginally less common
The only type of MP5 magazine that does not fit the CYMA is the SRC magazines, as they're proprietary to the SRC MP5
There are some pretty awesome MP5 body kits around like the Swordfish kit
There are also a few RIS kits around for the MP5
The V2 gearbox on a CYMA MP5 is very easy to work on
Has a nice metal construction which doesn't feel TOO bad- the plastics feel pretty good though
You can do the ultra-cool HK slap with the cocking handle!
Cons:
As it's CYMA, downgrading the spring power is required to make it's muzzle velocity site legal
The gearbox has potential but needs a little TLC to get the very best from it- a deep clean, regrease and gearset Angle of Engagement correction should be priorities in gearbox prepping
Some of the 'J' models of CYMA MP5 don't have a RIS system
All airsoft MP5s don't usually have a top rail for optics included with them- usually £15-£25 extra
Hicap magazines only hold about 200 BBs
Some people believe that because of the length of the magazine it's harder to use the weapon in the prone position- I had no issues with it, but that's just me...
CYMA MP5s are made with a type of pot metal that can be fragile under certain conditions- I had no issues with mine, but as it was my first AEG I wasn't exactly throwing it around
When firing the gearbox makes a horribly loud screech
The ROF is pretty low OOB
All in all I'd recommend the G&G GC16. It's a better all round gun which doesn't suffer from an unstable gearbox like the CYMA MP5 does (if it's stock that thing feels like it's about to shake itself apart when it's firing) with many more options for upgrades and external customisation.
If you're dead set on the MP5 design, go for the G&G MP5 instead. I've heard some very good things about them.
A quick note to any newcomers: 'What am bestest gun' threads aren't the best way for us to help you out when you're deciding on your first/second/whatever gun