I use both depending on the time of year. The 'full' sweatguard does a lot to help keep your defensive ammunition from tarnishing to quickly. That's the main reason for it's existance. It places a physical barrier between your sweaty, humid body and the gap between the frame and slide of the gun. Even if it's cool out and you're not sweating your body does give off moisture anyway and the less moisture that gets to your ammo the better. The biggestt downside is you have a rigid tab that can really dig into your floating ribs when the gun is out of the holster. This is particularly noticeable shooting from low kneeling around the left side of cover for righthanded folks (right side cover for lefties). Also for me, and my draw technique, I also can't start with my dominant hand thumb as high as I want it to be and have to reposition it before my left hand reaches the gun. If the thumb isn't moved it gets trapped under the left hand.

Here in Southern Arizona the coolest way I've found to carry a gun is with the gun/holster combo in direct contact with my body with a partially buttoned shirt as a cover garmet. In AZ's 110 heat the full sweatguard is practically manditory if you don't want to replace your chambered round, and possably 2 or 3 of the top rounds in your magazine, every few days if you spend much time in the heat at all.

The 'minimal' sweatguard is cut to the point it just covers the slide release, so there is still some protection from your sweaty body, but nowhere near what you get from the full sweat set-up. Minimal is far more comfortable when the gun is out of the holster and fits in better with my personal draw technique.( The technique I've found to be the fastest, and most fumble proof, is to jamb your thumb between your body and the gun and initiate the draw with a "pinch" of the gun with your thumb and the base of your index finger. The "gripping" fingers don't actually wrap around the grip until the gun is up from it's resting position in the holster by ~1". This greatly minimizes the chance of fouling the draw by grabbing clothing or fatty body parts. This technique only works really well with low to no levels of holster retention. But it works for me.)

What level of retention is best?

Unlike the Sweatguard retention is an individual preference. It also varies depending on the holster type. I automatically add a bit more retention to the OTP Pancake models. With the IWB and Appendix models if an empty gun is held upside down in the holster a slight shake will dislodge the gun when the holster is molded for "Low" retention. With the "medium" option the gun/holster combo will hold together when upside down and shaken fairly vigorously. Medium requires a slight tug to release the gun and has an audible "click" when the gun is fully seated in the holster. "High" levels off retention require a "Full, Firm, Firing Grip" and a definate tug to release the gun. I personnaly don't recommend "high" for a IWB and don't even offer it for the Appendix holster.

Personnaly, I have gone with virtuously "zero" retention holsters for over 15 years and the gun has never gone anywhere unless I wanted it to (IWB only). This included sparring, groundfighting, tumbling, and numerous gun retention classes (taught and taken) and the gun has never budged. Your ribs keep the gun in place just fine thank you. I once had a freind who was an MMA fighter who weighed ~280 lbs start with both hands on my gun, I didn't touch it at all, and he promptly threw me across the room. The gun stayed put. All it takes is to bend sideways at the waist and the gun cannot come out. You just have to be careful in public restrooms so the gun doesn't clank off the floor when you drop your drawers. I also take the gun off before getting in my inversion table.

All that said the VAST majority off people go with the "medium" option as they're conditioned to higher levels of holster retention.

I have X model Glock is it Large frame or Small?

All 9mm,.40, and .357 Glocks are "small" frame and everything from the 26/27 size guns to the 5" 34/35 models will work in the same holster. The "large" Glocks are all .45 and 10mm models.