If you don't know anything about a stand alone aside from you "heard about them" do yourself a favor and get the JWT ECU.
If you have a solid backround in tuning 3D engine managment, a stand alone will work great, but be forewarned, that if you don't know what you are doing it's super easy to turn your motor into scrap aluminum.
A good stand alone is going to run you about $1500 minimum, and then you will need a few hours dyno time at around $100 per hour so that has to be factored into cost as well. So figure about $2000 up and running for a basic stand alone.
If you get into a higher end stand alone you are talking $2500 just for admission. Sensors and whatnot extra.
there are a few guys in the d/fw area that know what they are doing, and their cars run really good, but like Jay said, if you don't know, stay away. I am just lucky b/c I have a friend @ Rotary that can do it, and know Trey @ Cobb Tuning, and he can do it, and both of them have hookups, so stick with the jwt, it is cheaper anyway. my $.02
stand-alones such as Motec, DTM and others can make your car running PERFECT with extensive tuning..however, like others said, it's expensive and probably not a good idea for a daily street car unless u see lots of road race action.
JWT is good enough