Definitely wait until you get the IS in your collection before you make a decision. The way you've got the Sideshow piece in that display looks fantastic.
If you can't keep both, you'll need to determine what your display preference is. In my opinion both are amazing statues. The Sideshow piece is outstanding despite what the rivet counters may try to make you believe.
I think the IS piece works best as a stand alone. Almost like a centerpiece. It's very action based but also quite low because of the crouched posture. It may look awkward paired next to museum style pieces like the SS troopers or the recent Boba Fett PF. I may display mine without anything else around it.
The Sideshow piece matches perfectly with the Boba Fett PF, and I think the two were designed that way for a reason. Because of the museum type pose, it may be easier to group the PF with the other pieces in the line.
Just my two cents.