My preference is complete games, to go all out with it, finish and select the next one. But if a ongoing gets my attention, I put it on a watchlist to have a look.
I usually wait for completion, but if a game looks really interesting I may play the first release I see. Very rarely do I replay to see the changes until it's fully complete though.
It depends on the game. NLT Games like Treasure of Nadia and the likes I like playing only after they're finished. Other games I will play while they are in development.
I'd prefer to play a completed game over an ongoing one, but honestly, if I enjoy, I'll play regardless. IMO, there's not enough really amazing completed games compared to ongoing/abandoned.