The biggest difference between the IR3 and LCD is the COPS. If you buy an IR3 make sure you get a Mem6 board, they're equipped with COPS II, plus that's the latest board revision. Also, in mem4 and up, you get the opto switch, which IMO is about 1 million times better than the standard microswitch (almost no resistence, much easier to get high ROF's). The IR3 is also about 1/4" shorter than the LCD, and a bit lighter, and the Fly IR3 is milled for less weight (don't buy the hype, it's not really noticeable, and even on a scale it's only like two ounces. It's still worth it because the cutlass guard is gone though) and it comes with a low rise feedneck. If you really want to make a big upgrade, take advantage of one of those people selling their A4 to buy a Fly4, and pick one up. With all the people upgrading, I'm sure you could find one for a good price.
