Polk Monitor 50s or 60s? Which should I get?

[kingtuna]

So I've narrowed it down to Polk Audio monitor series for my fronts. I'll probably get the CS-2 cause I watch a lot of movies. Now for my left and rights, should I get the Monitor 50s or 60s?

I don't know the dimensions of my living room, but to "guesstimate" it would be 20 feet by 15ish. I want something good, but not earth shattering because I have an ******* neighbor upstairs.

I have a powered sub, which I might also upgrade. Advice? Thank you.

Joker and others have been very helpful so far, and I appreciate that.

King TUNA
Sharp Aquos 46 in LCD
Sony DG-910
Onkyo HT-s40 7.1 speakers (keeping the rears, upgrading the fronts)
PS3 60 gb