Here's How You Decide on Which Edition to Run...
You need to make an opposed check for each edition.
1. Roll d20 (1d20 + the # of 4e supporters in your group + the modifers on the chart below = the DC of 4e.)
2. Roll another d20 (1d20 + the # of 3.5e supporters in your group + modifers on the chart below = the result of 3.5e).
3. If the result is greater than the DC of 4e, then you play 3.5e. If the result is lesser than the DC of 4e, you play 4e. Re-roll all ties.
4. A "Natural 20" always succeeds, regardless of modifiers (in the case of 4e)
5. A "Natural 20" for 3.5e also succeeds, but if another "Natural 20" is rolled the 3.5e players may upgrade to Pathfinder!

6. A "Natural 1" always looses, regardless of modifiers.
7. If both sides roll "Natural 1s", roll again

, but the DM has every right to subject all players to this: [ame=http://www.youtube.com/watch?v=B8LBpMuSTrQ]YouTube - DragonStrike[/ame]
Modifiers
+1 to +10 for bribes (you decided on how many points each bribe is worth)
+2 if a player can bring something "extra" to the table to aid your DMing (Cool minis, a battlemat, books already purchased for a given edition)
+4 if you are DMing at a player's home
-1 per gripe or complaint from any player about either edition (this is cumulative)
-2 per player known to have "attendance" issues"
-4 per player who is "rules lawyer"
-1 to -10 per player requesting bribes from you--depending on the nature of the bribe.