I would go for a RO/DI unit. Kent makes nice ones. They are relatively easy to install and worth the money. Get yourself a nice big reservoir rubbermaid tub for collecting the freshly made water.
Kent units are overpriced IMO. I think Aquarium pharmeceuticals makes a tap water filter that does a good job but it can get expensive replacing the filters constantly. The best thing would be to buy an RO/DI unit.
A tap water filter... although cheap up front will cost you more in the long run.
I would invest in a good RO/DI. I like the Typhoon III units for airwaterice.
