GBBR = less moving parts than an AEG
The break down of a gas rifle is simple but it is proper maintenance that requires the diligence.
A great working AEG will last you ages, you only have to worry about buying rounds and charging your battery. They are more reliable in colder weather and the preferred item of choice for people starting out, its also cheaper in the long run.
A gbbr requires regular cleaning to make sure everything is functioning properly. It does have the realism factor behind it but when you start to factor in propane/green gas, upgrades and after market parts, rounds, magazines that hold roughly 30-40 rounds as opposed to mags that can hold 60+. Magazines alone can run from $40 - $90 depending on brand and quality, AEG mags are far cheaper $10 and up depending on quality, brand, ect.
I have a gbbr scar, I love it but I highly suggest looking at some of the AEGs out there. I won't even comment on spring guns because most people will reiterate that you shouldn't be looking at a spring gun (l96,ect) until you get some experience behind you with an AEG.
You can get some amazing AEGs from the classifieds fully upgraded for less than what you're going go spend on a gas rifle but if you are set on a gas rifle I'd say go for it, I don't suggest the KJW, now that WE tech has m4s on the Canadian market it is a more cost effective platform to use and there are a variety of upgrades already available for both the internal and external of the gun.