I'm all for buying the GBBR to be honest. But I wouldn't recommend the KJW, it's more expensive than the WE M4... and the WE M4 is externally the same as a real steel M4...
Once you know the ins and outs of GBBR usage... they are about as real as you can get in Airsoft. They get expensive very face, at ~$50 per magazine, if you want to be sustainable in battle, you need a minimum of 4 magazines, at preferably 10.
They are cheap to maintain as you can rebuild the inside of a WE M4 for about $200 with upgraded parts, and propane is the only expendable other than BBs.
AEGs, well, I just don't like the sewing machine sound any more. The kick from a GBBR is a nice addition as well.
Sure, AEGs fair better in cold weather. They are cheaper to buy accessories for, and have tonnes of upgrade parts... but they are also finicky, far more expensive to repair (if you hire a gun doc) and if they go down in the field, there is nothing you can do until you have a nice clean table to work on them...
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.