Of the two, I’ve stayed at Venetian several times, but only visited the casino at Wynn. From what I’ve seen, I would prefer the Venetian. However, Wynn is closer to the conference at the Stardust. (The Venetian is next door to Wynn in the other direction, with a construction area in between.) Both are walking distance to the conference at the Stardust.
They’re both very nice, so you can’t go wrong.
As pokermonger says, don’t bother with a rental car.