I definitely think the try-before-you buy method is the best - not many people are willing to shell out $20 for something that may not be worth it (not to get off topic, but Dream Chronicles is a good example of the opposite to this - 30 min trial, 50 minute game!)
And I think ad-supported games wouldn't work for most LDW games, as they run in real time and I'm guessing I'm not the only one who would be ticked off by leaving a game running overnight only to find out that an ad interrupted 20 minutes in :s