Do you want rate-of-fire, or concealability? The G20 WILL be harder to conceal - my 3" SP101 has hidden on me in absolute plain-clothes better than a Sig P232...and that's with a $5 Bianchi holster and flimsy dress belt no less.
For woods-carry, where you won't need to hide it, you could go with the 10mm. It can mostly beat a .357 for sheer M.E. out of the 4.5" barrel, and the 10mm class is devastating for impact. The round was developed as a super-pistol round by Jeff Cooper IIRC, and the FBI developed the .40 because it was TOO powerful.
Rate of fire - when shooting, coming from semi-auto's, I DO rather miss the rapid-fire Semi's offer. However, it taught me to blow off $80 of ammo in very short time, with a bunch of little holes in the paper, rather than savoring my shots.
For Glock 20 owners - how is the reliability on your 10mm's? I know classically it's a pistol-wrecker (frame-abuse, slide abuse, etc.). That is a consideration - the SP101 was designed around the .357 Mag, and has been said to have a "K-frame chassis in a J-frame size".
Also, do you reload? It's MUCH easier to get reloading for .357 than 10mm, IMHO. By easier, I mean that you are more apt to find stray or extra .38 or .357 cases anywhere than 10mm. That goes for factory ammo, too. My local hardware store carries .38 special, and EVERY wal mart I've been into has at least 158gr JSP. Bullets themselves can be found in EVERY form, whereas the 10mm selection is a bit smaller selection. Lee sells the "hammer-it-in die kit" for .357/38 - NOT 10mm.
Also - Ammo cost. 10mm is a pricey round, no matter how/what/where. I don't know if it can run .40 S&W in the chamber ala .357/.38...or in the mag for that matter.
The other feature I enjoy of my .357 is the ability to shoot .38 - which, to many, is a fine little squirrel round - so I can handle small game AND large game with one pistol. Also - HUGE punchy rounds, unless you're experienced with them, WILL make you flinch. Flinching=bad shooting. After so many .357's, even when trained, your tired hands will start to want to flinch out of fatigue. Being able to pick up a cheap box of 130gr .38Spl's is great for this point in time...or if you're getting used to the gun, you won't be battering yourself or the gun.
I also must say that the Ruger SP101 has more character - the Glock is an EXCELLENT tool, but nothing more. It can be washed in your dishwasher, if needed. If it drops off the truck, big deal. But, there is a certain ritual and bonding experience when you get to break-down and hand-clean/polish all the spaces, spots, and crevices of your revolver. Knowing every inch of it like your own is a bit comforting. Plus, some of the grips you can get make a 3" SP101 absolutely GORGEOUS.
Where I got tired of striker designs/semis (Steyr M40, Walther PPS, Glock 19), was that when I was dry-firing practicing (ALL good shooting involves trigger control - something built by dry-firing), I had to re-cock the pistol after EVERY pull. This gets old...fast. Plus, there isn't much to a glock trigger - you pull it, its springy, it clicks. Or, gun goes bang, and hole appears in something.
With a revolver, there's that satisfying ratcheting as the next cartridge rotates to the forcing cone, followed by that sweet release and a hard THUMP to your hand - letting you know that you are only getting a taste of what just happened on the other end. If I want precision, I cock the hammer back, and I'm working with a rifle-grade trigger release. I can keep pulling the DA trigger over and over, and keep re-cocking the SA trigger with one hand. No more "Click. Rack. Click. Rack".
Also, with enough control, it's somewhat fun to "snap" your cylinder with the DA trigger (pull it far enough to move the cylinder to the next chamber, not far enough to fire - do when empty).
All that said, if it isn't obvious, I'm for the revolver. Yes, it only has 5 rounds, but really, if 5 .357Mags don't drop what you're shooting at, chances is it's on top of you already and your gun won't matter...
Just some food for thought...