I would stick with stock and buy from a good used parts dealer, such as Sandblaster on this site. If you contact him directly, he can save you some money on your purchase. I fried the coil on my 2003 SF conversion, and Sandblaster sold me a good used one that has been working for almost two years now.
As for the aftermarket coils, I've heard that they tend to fry plugs at an alarming rate. Sandblaster had one on his 500, and it didn't stay on for long. Come to think of it, I don't think I've read a positive review on them from anyone. Maybe that is just one brand and someone else has a better choice, but I prefer OEM.
