Hi there. I've had my Bushmaster XM15-E2S for a number of weeks now and I recently began to wonder about new upper sections. I love shooting .223 and all, but I was wondering what the benefits of a different upper are (aside from different calibers). Is there a good caliber to convert to? Are switching uppers difficult? I've looked at some 7.62X39 uppers as well as some 9MM uppers. Any advice or websites anyone could point me to would be greatly-appreciated.

switching uppers is the easy part.  Two tack down pins and you're done.  The hard part is trying to control the urge to buy a lower for the new upper.  You'll eventually end up with many rifles. Resistance is futile.

I'm having trouble deciding between a .22LR upper and a 9MM upper. 9MM seems much more expensive but has larger capacity magazines to shoot. Obviously shooting .22LR is much cheaper....Anyone have any suggestions?

I say buy both. If you don't want to do that, you can buy a .22 conversion kit for the .223 upper and then spend the big money on a 9mm one.