I think that's where the quality of your sub comes into play. What kind do you have? Dave will help you with the recommended crossover point so if you listen to music in 2.1 rather than 2.0 the sub will take the deeper bass and the Luna's will handle the upper bass on up to the highest treble. I agree with you that if you listen to music without the sub you might want to go for the Sierra 2's.

As far as future proofing, if it were me, I would get the Luna's now with a Sierra 2 center. After you move and presumably have more space, then go for either the Sierra 2's or the towers for your mains and move the Luna's to surround duty. That would be an amazing sounding system.