I own a Beretta 92FS and a G19 (non-X) although my G19 is fairly extensively modified at this point. The biggest difference between the 19 and the 19X is grip length, they both share the same barrel length, but the 19X uses the grip length of the 17.
Both are excellent guns in their own right and to be honest, you wouldn't be wrong to get either one. Both are going to be more than accurate enough, and more than reliable enough for whatever you need, so I'll call those a wash between the two.
Glock 19: There are a lot of Glock haters out there but the fact is, they are actually pretty good learner guns because of their simplicity. I carry my Glock 19 concealed every day, and compete in USPSA (B class shooter) carry optics division once or twice a month, and shoot about 5k rounds a year through this particular Glock 19.
Pros: Very simple, there are no frills with a Glock, which is nice for new shooters as there is simply less to keep in your mind. The aftermarket for Glocks (see above) is second to none, anything holster, mag carrier, sights, barrels, mag releases, mag wells, triggers, anything you want to change, you'll find a dozen groups making products for it. I'll even contest that the striker fired single action only trigger is better for a new shooter who is going to carry the gun. IMHO the perfect size gun for someone who is going to own only one handgun, on the large size for CCW but still doable, perfectly at home on a competition range, and fits the home defense roll just fine.
Cons: It's not sexy, at all. Not as smooth as a Beretta 92. Maybe a little more snappy than a 92.
Beretta 92fs: I love my 92fs, I'll never sell it. It's a much different experience than the G19, It's the only gun I own where I can feel the slide reciprocate when fired, I'm very aware of all the mechanics of the firearm as it goes off. It's kind of hard to describe. I searched all over for a 92 Inox "Ghost" (stainless trigger and safety as opposed to plastic) and am very happy that I found one. Mine is a range toy, and occasionally gets brought out for a USPSA Production run. I shoot about 500-1000 rounds a year through it.
Pros: Very very smooth shooter, after a couple thousand rounds, you'd swear the slide is on bearings. It's an inherently sexy gun, I know that's just an opinion but I'm putting it in the "Pros" column anyway. I do prefer the alloy frame and steel mags of the 92 to the polymer frame and polymer mags of the Glocks.
Cons: I'm just not a huge fan of double/single guns, especially for a new shooter; they're just a bit more complex to learn. For what it is, it's huge; I know the pic makes them look similarly sized, but trust me, they are not. For a gun in which you wish to carry concealed, this would be much more difficult with a 92 than a G19. I dont know if you care or not, but the 92fs' do not have a rail, so no weapon light which I consider to be an absolute must on any defensive firearm; but the M9A1s do have a rail, so that might be kind of a moot point anyway.
Conclusion: Tough call. If you're only going to buy one handgun, I have to recommend the Glock over the 92fs; but if you plan on buying more as time goes on, buy the Glock, then a 92fs. I love my 92, I really do, but I do find myself shooting it and enjoying myself, then going right back to shooting the G19. I shoot almost an order of magnitude more ammo through the G19 than the 92fs, it's not that the 92 is a bad gun, it's certainly not, but the 19 just strikes a balance that it does everything pretty well.