An MP7 would do just fine or as MigaFox suggested a G18. It does not have to be full-auto to be a sniper side-arm. If you really want a AEG, go with the MP5, but a semi-auto pistol will do the job just as well.
To answer your question, I would recommend an mp5 without the stock (I believe that makes it an mp5k) for it's weight and rof. My friend had one as a primary and it shot very fast, but didn't have the range (which shouldn't matter for you since this is a secondary.) I can't say anything about the g36, but I have had the S-system in the past and for some reason, I didn't like it. I guess I expected more out of it and I wasn't familiar with m4 upgrades/internals at that time, but I used mine as a primary and owned the long version. Hope this helps!
A good sniper rifle. Stealth is your primary weapon.
A more serious answer is:
A really good spotter with excellent camo and fieldcraft watching your back.
Both of those will do a better job of staying in the game than any aeg.
To actually answer:
If you carry a full size aeg you limit mobility to an extent and you are conflicted. Do you use your aeg or your rifle? I would use a decent pistol you can draw in around a second. When you NEED the secondary you will often need it NOW.
You could use an aeg to move around the site and a basr 'in position' but I think you'd be better with a DRM.