That is a decent gun, if you must have an M4 and you want it with a metal receiver. Out of the box it will not disappoint compared to anything you'll be up against skirmishing. Bear in mind that paying more money for 'pre-upgraded' guns doesn't always make sense, because they may not be the upgrades you would choose. It's also worth considering that right now you don't know much, so trying to pick 'the best' is not going to be easy - you don't know what will suit your play style best. The GC16 Raider S will work fine as an all rounder and has rails all over it, so it'll be a simple enough job to add accessories as you do develop your skirmish techniques.
There are differences between gun lengths, but TBH I CBA to get into it right now. If you decide later that you would rather have 6" more barrel, it's a simple enough job to fit a barrel extension and longer internal barrel and it'll look fine. If you stick with short and want more power, a 233x6.03mm tight bore barrel should just about scrape under 350FPS without needing to change the spring.
You may have already read about hop up, but if not stick around and you will. The G&G stock set up is good as it is, but for additional accuracy you could upgrade to an H-nub.
If you have large hands you may find the controls of a stock M4 a little awkward, but this problem can be ameliorated somewhat by fitting a pistol grip with extra width to the backstrap. G&G make one which also has ergonomic moulding. I forget what they call it, but 'tactical' will be in there somewhere...
Of course, if you had any taste or good sense you'd be after an AK, but the answer to the basic question of "is this gun any good?" is yes.