From a primarily-vocals standpoint, yes, the RB3 versions are clearly superior. If price isn't an issue (is the pre-RB3 sale still going on?) you're going to have a better time with the RB3 versions.

I wouldn't worry too much about who owns what online - most of the people online will not match up with much, if any, of your DLC unless you're one of the crazy people like me that owns virtually all DLC. Amongst random online players, I rarely see Queen songs show up as playable, let alone picked.

Metallica, Disturbed, Avenged Sevenfold and Hysteria by Muse are likely to show up. Consider yourself lucky if anything else that isn't an export appears.