We just purchased (and love it so far!) the Core set and a YT-1300 (just because we like the model). Next up should be some Imperials to balance things out. I guess it's pretty much a points thing at this point yeah?
For example we should get something that gets the Imperials close to Luke+Falcon point level, but the points values aren't published anywhere are they?
I like the idea of another Tie Fighter or two (for example Interceptor or Advanced) but without knowing the point values it's a guess.
I guess the real answer to this question based on reading some of the other posts is really "Whatever's available." :-)