RE: What one?
I suggest you get the Mini-Z because they are so much fun. The HPI RTR Micro RS4 is faster then the RTR Mini-Z out of the box but do to it's size it's harder to play with indoors. So, if you plan on getting something to play with inside get the Mini-Z if you want something to play with on a smooth cement track I'd suggest the HPI, but you can play with a Z on cement as long as it's pretty smooth. I have two Z's and I love them so I think you should get a Z. Oh yea, if your worried about the speed of the Z it's about 12-13.5 mph out of the box depending on what batteries you use and the RTR Micro RS4 is 13 mph on 4 AA alkaline and 18 mph on Trinity's 6-cell NiMH pack. Hope this helps.