Having owned both, I think I can give a more detailed, more useful answer for you than some (not all) of the other posts.
I bought a Kel-Tec P11 as my first gun, ever. After a while, CCW passed in MO and I got one. I carried it for about a year and a half until I got my Browning Hi-Power (better job = more money) and the P11 was moved to backup duty.
Since then, I have acquired guns including Glocks. I eventually traded my P11 for a J-frame revolver.
The Kel-Tec is better if you can't afford the Glock. This is not a knock against the Kel-Tec; I could not have afforded to carry a gun for two years if Kel-Tec hadn't been there for me.
The Glock has less recoil. The P11 recoil isn't bad, in my opinion (but the recoil in a Glock 27 IS bad) but will be less pleasant for a range fun gun.
The P11 is smaller, and the non-square slide conceals much better in a pocket, and takes up less room in your waistband.
The Glock is built better for durability - the P11 will eventually wear out, but the Glock (with routine maintenance including recoil springs) will last a lifetime and longer. This is why I traded away my P11, I was going to wear it out and I would rather have a gun that can be shot frequently and voluminously and still be passed down to my children.
The P11, being priced lower, may require some finishing operations that were performed at the Glock factory. I understood this when I bought my P11, and I used some fine sandpaper and polishing compound to do the work that didn't cost me that extra $250. This is, in my mind, acceptable for someone who needs to save the extra cash and knows that it is necessary.
If you intend to shoot it a lot, and you can afford a Glock, there is very little reason to get a P11. The little bit of extra concealability probably doesn't make much of a difference - if it does, please remember that the P11 won't take the volume of shooting that a Glock will, and only shoot it enough to remain expert in its use.
The P11 holds a special place in my heart as my first gun and my first CCW, just like that pickle-green 1980 Plymouth Gran Fury holds a place in my heart as my first car. Now that I can afford better, I own better. But I remember where I came from, and I remember what got me to where I am.