Yes and no on SNs as they still mfg Gen 3s even though Gen4s have been around 5+ years. This is partly because Gen4s are illegal in CA.
Gen 1s were mfg'd from 1983-88/very early 89 (1st U.S. Imports were early '86) and were mfg'd as the 17, 17L, and a very small # of 19s and have the pebble grain wrap-around texture on frame, thin "pencil" barrel, no rail. It is estimated that fewer than 200 Gen 1 19s were made and most were purchased by the Kansas City Dept of Alcohol and Bev Control and are marked as such on the slides.
Gen 2s were mfg'd from 1989-98 and were mfg'd as the 17, 19, 20, 21, 22, 23, 24, 26 and 27 (though these "baby Glocks" are sometimes called Gen 2.5 as they had thumb grooves in frame), 31, 32, and 33 (also sometimes called Gen 2.5). These guns have checkering on front and back strap of PG (unlike Gen 1), no rail, or thumb groove with the exception of the 26, 27, and 33 subcompacts. Also- there was a small overlap late in Gen 2 production that was kind-of a transition and a Gen 2 would have a thumb groove in frame like a Gen 3 but no rail. If you've see. Hickok45s vid on YouTube his earlier Glock 21 is one of these. Gen 2s had a 2-pin frame with the exception of some of the 40s and 357s which required 3. The Gen2/3 overlap occurs in the CXX-SN range during the middle of 1998.
Gen 3- 1998-present. Mfg'd in all models with the exception of the newer 41, 42, and 43. Thumb groove in frame, accessory rail, finder grooves on PG. From 2008-late 2010 they offered an RTF2 (Rough Textured Finish #2) frame that some felt was too aggressive on the skin so they went away from it. Additionally many of these RTF2s had "fish gill" slide serrations where they serrations were angled instead of straight up and down. The RTF2 option was offered in the 17 and 22 (most commonly), 19, 21SF (short frame version of the 21), 23, 31, and 32. RTF2 was phased out around fall 2010 although the Larry Vickers limited edition 17s and 19s have the RTF2 frame. Gen 3 saw the intro of the 29, 30, 34, 35, 36, 37, 38, and 39.
Gen 4- Introduced at SHOT show in Jan. 2010 frame has a modified RTF finish that isn't as aggressive as the RTF2. Interchangeable backstraps, tell-tale "Gen4" on slide, "MBS" stamped on slide indicating Multiple Backstrap System. All models are manufactured in Gen4 with the exception of the Glock 36, 37, 38, and 39. Gen4s have the 2-piece recoil spring assembly (like all the subcompacts have) to allegedly dampen felt recoil.
Sometime between 2007-10 they went away from the Tenifer finishing process (Tenifer is NOT a finish) and there have been wide variances in the finish ever since from the smooth dark "frying pan" finish that has the texture of a cured skillet to the chalk board finish that is light gray and tends to skuff easily.
I'm likely forgetting some things but in a nutshell those are the differences. As far as collectibility goes, the Gen 1s have become collectible particularly the 19s as they're remarkably rare. The Gen 2 357s are kind-of collectible as well as they were intro'd very late into Gen 2 production and there weren't very many made before the Gen 3 transition. Gen 2 357s were marked "357 SIG" and early 40s, "40SW" before Glock dropped the company names of SIG and S&W from their slides.
I'll stick with my Gen 4. 
