I'd take Nexus 1. It's still the only officially sanctioned Google phone, and Android 3.0 will probably be available for it first.

So far Nexus One is always the first phone to get updates. E.g. Eclair (2.1) and Froyo (2.2) were offered to Nexus 1 first. So it's not unreasonable to think that 3.0 will go to N1 first too.

Some people say current phones including Nexus One and Desire cannot support Android 3.0. But Google has said this is not true.