I'm gonna talk about the G36 and the combat machine in this to hopefully give you an idea
Assembly/dis-assembly
I've just sold a G36C and bought a G&G combat machine - both mentioned above as the go to starters. The G36C is incredibly easy to take apart and reassemble, it is either a pin to pop out (for example, popping out one of the pins will let you take the handguard off and replace the battery if it is electric), or maybe a few hex screws. It was my first gun that I had ever taken apart, then I just moved on to disassembling a Tokyo Marui G18C - few more parts, but you get used to it. Anyway, in regards to the G36 vs Combat machine, the G36 is the easier one (in my opinion) to take apart if need be. However, that being said, I like the CM16 more (faster ROF, quite accurate, very customisable due to it being an M4 platform).
Performance
Now, obviously you want a nice solid platform to start off with. Both of these hold their own, and comes down to preference. G36 = Slower ROF with quite reliable accuracy. CM16 = Fast ROF, but still retains great accuracy. But you shouldn't be running around going full auto all the time anyway, you'll just eat through the BBs and it is pretty unnecessary. Semi-auto does the job most of the time. I had no problems with the G36 for all the time I had it which suggests it is quite good, and even if there was a problem, it is easy enough to take apart and replace parts if needed. I haven't had much experience with the CM16 (I got it yesterday), but it is second hand and seems to be in great shape.
Ergonomics
G36 is quite comfortable and easy to hold. Doesn't feel clunky or anything and has a nicely shaped handguard. Got some RIS rails at the end of it too, if you buy the G36C (shorter one). The grip is just smooth polymer, so it might not be the best to hold, but it doesn't cause a problem. Folding stock is great too, which allows you to adapt depending on your environment. Top rail seems really nicely positioned, and would work great with an added optic. You can get the G36K which has a built in scope, but I've never personally used it. As for the CM16, I have a Knight's Armament quadrail handguard, which isn't the most comfortable to hold - some kind of a foregrip as an accessory would be nice. It has a more ergonomic grip too, which sits nicer in the hand. The stock varies on the model, but I have a G&G GOS V1 retractable stock (comes standard on the G&G GR4, but can be bought separately) which is much more comfortable than the standard one I had. Oh a bonus that the CM16 has is that you can change the stock position to make it shorter or longer, which you can't do with the G36 unless you replace the stock to get one designed for that. Top rails are a little lower than on the G36, which will take me some getting used to, so I can't comment on that yet. For me, the G36C is more comfortable, but the CM16 just feels better to hold (if that makes sense).
Customisability
Well, to put it simply, the CM16 has a lot more options than the G36. The M4 is a very popular and widely used platform, and so there is a lot for it. G36 doesn't have as many parts available. Each can hold your standard attachments though, like an EOTech or a foregrip. Also, Pmags for the M4 seem to be everywhere - finding them for a G36 is a different, expensive story.
So, if you want an easy to maintain gun which feels quite comfortable, shoots at a decent ROF with good accuracy but doesn't have many customisable options, then the G36 would be a good choice. If you want something highly customisable, maybe a little harder to maintain (for example, taking the barrel out of a G36 is so much easier than a CM16), fast ROF with pretty good accuracy and still quite comfortable yet may require accessories depending on your parts, then the CM16 is a good choice.
Personally, I prefer the CM16. Both are inexpensive and great starter choices.
Maybe shop around on these forums or Zeroin and check their classifieds for second hand guns - you can get some amazing deals.
Sorry for the lengthy reply, got a bit carried away.
That is all my personal opinion, some may disagree which is fine, but do not read that as factual - obviously different people have different preferences.