i have both nero and imgburn and i use imgburn for wii & gamecube games because nero won't let me burn at a lower speed with my media type and burner, nero only lets me burn at 8x and imgburn lets me burn at any speed i want.
actually, depending on your burner and media, you can't burn at slower speeds. with imgburn, even if you set it to 4x or less it can only burn at your burner/media combo's compatible speed.
In my case, my burner won't burn 16x verbatims at speeds less than 6x (only burns at 6x, 8x,12x and 16x), so even if i set imgburn at 2x it will cheat and burn at 6x instead. just take a look the next time you burn at the data transfer speed as you're burning.
On topic, I used to have nero for general use and imgburn for isos. but now I've found the wonders of burning studio (Ashampoo) and I've pretty much forgotten about nero (bloated piece of crap now) and imgburn (still have alcohol for virtual drives though). btw, my burner seems to work better at 8x/12x