Ok the sub I have now is a 12" downfiring sub from Best Buy, I just bought a RSW 15 off of ebay for $750 (not sure if I got ripped or not) but am I going to hear a big difference between the two? Should I hook up and use both subs or would I be better off with the one 15"? Thank you

If it were me, I'd hook up both and play around with them, unless you have another system you could use the other sub for.  Setting up two subs can be tricky and timely, I run two subs and have them configured as I like, it works awesome.