Something I learnt was it comes down to what you like personally more than anything else. If you stick to a good manufacture then your 80% there.
I'm not a fan of m4's so I have a scar h and vector downside to this is not many others do so I'm normally the only guy with vector mags and scar h mags so if I have a prob I'm on my own.
The great thing about airsoft is you can be totally unique.
The 551 is lovely if you like it great go to a shop hold one and see if you really like it.. then off you go.
Most rifles can be used in cqb even the scar h I have at worst the stock can be folded.
Basically I'm saying buy what you want and like the look of make sure it's a decent manufacturer and do some online research on your chosen gun and violla.
I researched the scar h for 4 months before I bought it ( only recently ) and the vector I bought because I love them.