I own an RFB, never had an issue that wasn't caused by starving it for gas. The gas system needs to be adjusted every time you switch ammo but who cares? How many different types of ammo does a guy use in one rifle? Once I find a handload or a factory load that works well in a rifle I stick to it. It's not like your going to be going out hunting and switching between three different types of ammo a day, and if you do then you only need to figure out at an earlier time how many clicks +/- it needs for each load just like you will need to adjust your scope when you switch ammo. It really isn't the big deal everyone makes of it. Once you understand the gas system it only takes a couple shots to get it set close enough to function reliably.
As for milspec, well if you do a little research the KelTec is built from some pretty high end metals and while everyone seems to like to bash it as a "plastic" rifle they obviously haven't taken one apart since everything that needs strength is steel.
Battle proven? I know the Tavor is in service with a few military groups which does say something for it but what battles has it actually been in?
Madmardigan, As for your comments about lifespan, 15000 is about half of what a good quality chrome lined barrel should get but there is a lot more than just barrel life to consider and since most rifles in military service aren't going to go their lifetime without maintenance and refitting from the unit armorer it really doesn't make much difference in the big picture comparing those rifles to what we own. Like you said though, most of us won't put nearly that many rounds through a rifle. Just look at the EE for proof of that as almost every rifle for sale claim to have only 50-100 rounds through it

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I'm in no way bashing the Tavor as unfortunately I've never had the chance to fire one and everything I've read and heard about them leads me to believe they are high quality and I'm looking forward to trying one.
I'm just a little tired of all the KelTec bashing as I own an RFB, KSG and sub2000 and have had zero issues with any of them and while they may not be for everyone they are good firearms that work. The constant bashing because of the plastic makes no sense either since almost every rifle and pistol out there these days has a good percentage of "polymer" and all it does is keep weight and price down so to me it's a good thing.
To the OP, the two rifles are completely different other than they are both bullpups so you need to decide if you want 308 or 223 and after that there aren't many options.