1 Which is more complex (rules)?
2 Which is slower?
3 Which has most replay value?
4 Which has the least errata?
5 Is this good with 3 players?
1. Onward is more complex.
2. Slower or longer? The Second Crusade is certainly shorter than a normal game of Twilight Struggle, but the other scenarios are longer. Neither game is slow, though admittedly bringing Barbie's massive force through Asia Minor can feel brutal if it's not done slowly.
3. Replay value? Both are excellent.
4. The new version of Twilight Struggle should be pretty much errata free. It's the second edition.
5. Onward is multiplayer. Twilight Struggle is a two player game.
Honestly you have 2 very different games. I would never give up either one. ;-)