My vote honestly would be exhaust, I honestly can tell you I am not a fan of the exhaust myself, it almost sounds like your car is broken =(. I think there is something bad/not good about the x-pipe. I say you buy the system U exhaust, not just the catback but the midpipe and catback and dump the current x-pipe.
i mean you couldnt pass up the deal on that x-pipe, it was so cheap, you had to try it, its not like you took a terrible hit if you decide not to use it. Hell you could probably sell it for 200 and make all your money back on it.
Plus I just ordered my rims, wouldnt want to take away from the beauty and admiration mine will get if you get yours at the same time =P