you don't really want to mess with a guitar amp much. Besides, the bass will sound terrible on it, especially the kick drum.
I have owned both of the Simmons amps. I am very impressed with them, especially considering the price. I got the 50w Simmons and later upgraded to the 200w when I saw it used on CL for $175. The 50w leaves a lot to be desired in terms of total quality. The 200w is much better. It has a sub, 2 mid range speakers, and 2 tweeters. It has a lot of functionality in terms of 1/4 and aux in. You can independently adjust the bass/treble/mid/sub, and then also adjust the drum and input sources so you can balance the Mp3 player and drum kit. It also has outputs for a PA system.
I have used the 200w to play gigs. At 50% volume it is loud enough to play outdoors with a full band and doesnt lose any sound quality. If you are playing a bigger room, you will probably just plug into a PA anyway. Even the 50W is more than adequate to play with some friends at home.
I looked at the Roland unit you mentioned, but it was just too expensive. I thought the Simmons did very well, especially considering it is half the price of the Roland kit. The nice thing about the Roland kit is the high range speakers are separate from the kit, so you can move them around. But in the end, I would rather have them all in a compact unit, like the Simmons.
Overall, I would only suggest shelling out the big bucks for the high end stuff if you are a total audiophile. But then again, why not just get a nice pair of headphones for a lot less?
Here is the amp I am talking about
FYI: I use a Roland TD-4 ekit