Nimh are the better batteries to get. They have less "memory" and you can get them in larger capacities. I believe Nicads are only available up to 2400mah while you can get more than double that with Nimh. Just make sure the charger you're using can charge nimh batteries before you buy.

Lipo is great but there is much more to consider than just saving up for the battery. You'd need a charger and should run an ESC that has a low voltage cutoff specifically meant for lipos.