Total editions...
WEG (all editions) is a d6 dice pool, roll and total. It has (effectively) 6 editions...
1E: No initiative*, no haste. Everyone declares, then everyone rolls the actions, then actions resolved in descending die-roll order. No full round defenses. TN's in fixed increments of 5. 2 scales, ships and characters.
1.1E: Core plus rules upgrade. Adds full round defenses. TN's now ranges peaking at increments of 5.
1.5E: Core plus Rules Companion. Includes rules upgrade changes. Restricts force powers, adds haste, adds dice-capping for scale, has 6 scales.
2E: Adds initiative roll, alters process for full round defenses and haste, adds wild die, changes scaling from over cap=0 to over cap=cap. (Making affecting out of scale actions somewhat easier.) Movement ratings go from dice to fixed values.
2.5 aka 2R&E "Revised and Expanded" AKA 2RE: further changes to full round defenses. Scaling simplified and changed from dice capping to bonus dice.
3E: never released as such, but later released as "D6 Space"... mechanically, 2.5 with added advantages and disadvantages, but without the SW IP...
Note that no revised corebook for 1.1 nor 1.5 was published.
A fan made 2.75 level has been released in violation of the US laws...
License revoked for cause in mid-late 90's.
WEG Live Action
no-touch LARP rules derived from 2R&E. Much simplified.
WotC
all 3 Wizards editions are part of the D20 system; all are roll 1d20+stat+skill for TN+.
D20 Star Wars - essentially 3.5, but adding the Vitality/wounds split in place of HP. Force powered by burning vitality. Most non-crits do vitality damage, too. Despite this, Jedi get a pretty strong mary-sue type feel.
D20 Star Wars Revised - many minor changes. Not much improved, tho'.
Star Wars Saga Edition - major overhaul - skills simplified to booelan (you have them or not), Force powers reduced. Mechanically fairly similar to the (later released) 4E D&D.
SWSE vs WEG
SWSE is noted for massive fan uptake - but many continued to use the out of print WEG edition. Fan demand for WEG so high that almost all the books have wound up pirate-scanned.
Now that SWSE is OOP, most of its books scanned as well. The two largely don't overlap in fan base; SWSE fans tend to not be d6 fans, and vice versa.
FFG
3 game lines using one engine. In order released....
- Edge of the Empire - aimed at, essentially, "Firefly in the SW universe"...
- Age of Rebellion - aimed at "active duty in the Rebel Alliance."
- Force and Destiny - Full up force users - primarily jedi.
VERY minor revisions to a few things in AoR over Edge; I've found no such changes in F&D over AoR.
All use the same "funky dice" - color coded (but shaded such that the colorblind simulators leave them all distinguishable with any of the 4 "common" forms of colorblindness, including achromia. Symbolic results and cancelling of symbols is simple enough for even 10-year olds...
All three lines have a beginner box with no character generation, slightly lighter rules (crits resolved differently), a "teach you how to run the game while playing it" adventure, and 4 pregens in box, plus 2 extra available as PDF, and a follow-up adventure in PDF.
All three have their own corebooks, as well.
There is some overlap of fluff/prose, but it's not the same in the corebooks.
The vehicles and adversaries both have about a 1/5th overlap between any two cores; only a handful of items are in all three.
While the rules are largely the same, each has different classes, different specialties (with some overlap between AoR and Edge), and a different driving score.
- Edge's driving value is Obligation; when it triggers, it's bad, and requires you to deal with it, or you suffer penalties, and it may go up. Further, it can go up due to player action. Deal with it the right ways and it goes down.
- Age's is Duty and Contribution Rank. Duty is a positive value, when it triggers, it is good. It's raised by accomplishing missions. Contribution rank is gained by making duty exceed 100...
- F&D's is Morality. Follow the code, it goes up. Break the code, it hovers around a central point or starts to drop. Low value is bad, results in falling to the dark side. High values good, result in becoming light side. When triggered, amount of change doubled, be it for better or worse.
FFG vs SWSE & WEG
I'm noticing a lot more overlap between FFG fans and d6 fans than SWSE vs d6. Many SWSE fans seem to dislike the dice enough to stay with SWSE.
FFG Force powers are more narratively powerful, but mechanically more balanced, than any of the others.