The big difference between the G21SF and the original G21 comes in the ergonomics category. The frame is just enough smaller that it fits everyone a lot better. Lots of small-handed shooters have passed on the G21 because they couldn't get a proper firing grip on the large gun. The newer G21SF fits smaller hands much better. What about us big guys? I'm 6-foot-4-inches and weigh close to 300 pounds, and the G21SF fits me a lot better.
Several of us did a side by side comparison on the range of the G21 and the G21SF. Drawing and engaging multiple targets with multiple shots gave us a pretty good idea of the difference. At speed, I couldn't get the G21 lined up properly in my firing hand. Instead of having the center axis of the bore in line with the bones in my forearm as it should be, the G21 consistently ended up slightly off center and to the right. This was not a problem with the G21SF. It indexed properly, making for faster times, better control of recoil, and increased practical accuracy. While it seems like a minor detail, it makes a huge difference on target.
The G21SF's polymer frame makes it easy to carry due to its lighter weight (28 ounces). Add to that the ease of maintenance of the polymer frame and the Tenifer finish on the slide and you've got the complete package.
There was just something about how the slim-frame Glock handled the big .45 ACP cartridge. Even when firing Federal 230-grain +P Hydra-Shoks, the recoil was more than manageable. Fast, accurate shots were no problem. Double Taps, Hammers, Zipper Drills, any non-standard responses were completely controllable.
Let's take a closer look at the Glock 21SF. Similar to a standard G21, the G21SF has an overall length of 7.59 inches and a height if 5.47 inches. The width (1.27 inches) and the barrel length (4.60 inches) are also the same. They both carry 13 rounds in the magazine, but the magazines are not interchangeable. The G21SF uses a different magazine to accommodate its ambidextrous magazine release. Aside from the shorter frame on the G21SF, the magazine release on both sides of the frame is the most notable feature.