The Glock 26 and Glock 39 are identical in size. (Feel free to look them up on Glock's website to verify.) The Glock 26 fires 9mm rounds, and comes standard with 10 round magazines. The Glock 39 fires .45 GAP rounds, and comes standard with 6 round magazines. According to ballistic tests, the .45 GAP and .45 ACP have identical ballistics at the common grains, the only difference being that the GAP fits in smaller-framed guns.
Ammoman.com will give you 1000 rounds of .45 GAP FMJ (for target practice) for $350. You can generally find decent hollow points online for the same price as .45 ACP hollow points.
The Glock 39 is as small as the smallest sub-compact (9mm) Glock, fires the round with the most stopping power, ammo is readily available at good prices, and the .45 GAP has recently been chosen by many of today's law enforcement departments, as well as the USMC.
I was really leaning toward a Glock 26 until I discovered the new .45 GAP and Glock 39. Assuming I can handle the larger caliber without any loss of accuracy, can anyone think of any reason *not* to get a Glock 39?