Well, an SXR Advent ignition has 4 curves to choose from instead of having only 1 like the MSD superjet enhancer, so "technically" the SXR Advent setup is a better deal.
Also, RD Heads for an SXR Can be bought for $242 (with an optional girdle kit for I believe around $80?) which is cheaper than the JSU sxr head, an also the yamaha head. I believe the RD head for an SXR is the cheapest one out there, and is a great head.
The pipes are the only two things compared that are really accurate, since the SXR only has one wet pipe offered by the aftermarket. Im not sure if there are more wet pipe options for superjets, but the B pipe is the way to go.
The surplus and variety of superjet parts for freestyle is quite larger, and the superjet is alot more nimble for freestyle as you can tell. The SXR has great potential, but can cost a bit more because of the lack of options for freestyle parts. And Superjets can be found used, with tons of mods, for a great overall price, which is another main reason they are so popular for freestyle.
By the way, I have both an sxr and a superjet. They are both fun. But if I could only have ONE ski and HAD to freestyle, I'd take the sxr and put a ton of money into that, instead of spreading out a ton of money into 3 different skis. 2 sxrs and one Superjet that I have.
You really cant go wrong either way you look at it.