There are several schools of thought regarding this topic, but namely:
EBB- Either piston drive or Pneumatic, both driven by a battery. Mechanical, or piston driven, is a very primitive way of doing it, it started as a homebuild, and was ported to the market. These add stress on the internals, and generally just throw an un-needed hassle into the gun. These guns are usually very cheap, both to make, and in quality. It is cool, but I would avoid them.
Pneumatic Blowback looked to solve this by not interfering with the piston. Again, it adds another part into the system, however, while it does not directly interfere with the works, you must be mindful of the unit when upgrading. It is an external part on the gearbox, and takes some extra work, and care, to remove.
I own one of these, and, while I can't say I use it as a primary, I really do enjoy the blowback feature, it just makes the shot really satisfying, and lets me get some vibration feedback for each shot, to help me out a bit. It really does add something. Took me a while to get used to a normal AEG again
GBBR- These are purely gas driven systems, done a variety of ways, and honestly, in my opinion, really not worth the money. I have owned, and sold, a ton of GBBRS, two pdw's from WE, a WE M4, WOC, and a variety of SMG's, such as my mac 11's and mp5's.
There are a hassle to fill, a hassle to maintain, and honestly, I was afraid every time I shot mine, that they would break.
If you ever need proof that GBBR is a love hate relationship, go and look around on forums like ASF, ASR, and ASM, the only thing more available than advice, is broken GBBR parts.
That said, the recoil is cool, and the range on these guns is better out of the box, then out of the box AEG's (Usually) But, the gas in mag system just provides too much inconsistency for any type of accurate use. Even with the best RA-Tech parts, and the most sealed mags available, stuff still breaks, and problems occur. Taping a mag for external gas can solve this, but then you lose the effect of having a replica with changeable mags, and you have to carry a hose.
Also, GBBR mags are a racket. For there quaility, they shouldn't cost $30 and up.
In short,
I love despise mechanical blowback for being cheap and cool
I love despise Pneumatic blowback for being harder to work on and awesome
and
I love the recoil of a GBBR, and managing my gas and low ammo (don't judge me lol), and really despise there high cost, high maintenance, lack of reliability, higher repair cost, inconsistency, and everything else about them.
Choose wisely
*NOTE: I have not yet tried KWA's new GBBR's, so, do internet research on them. I also STILL intend to buy there Kriss, even with my thought on GBBR's
