Xbox 360 HD DVD Drive or Rock Band?

I am in a kind of predicament here. I have just gotten an HDTV ( that looks awesome ) recently. I have a PS3 and watch Blu Ray movies. I want to get the HDDVD drive to hook up to my 360 and watch HDDVD's and be fully up to date with the new movie formats. However, I also really really want Rock Band. So simple question here: Buy 360 HD DVD Drive and a copy of Transformers on HDDVD. Or buy Rock Band. What is the better purchase?

Obviously rock band. The hd dvd drive isn't worth it. Especially if you already have a blu ray player. Citan76

Isuggest getting rock band, no need to waste money on a HD DVD Drive if you already have a ps3.
This can also be simplified to which one would you use more. Do you game more than you watch movies? Or do you do the opposite? I think after you determine that, the answer will be simple.