if you're always upright the stargate style one works just fine, but becomes an issue when you go prone.
If you have the extended buttstock then the stargate style loop around the body will be harder to do I think. It needs to be super tight so it doesn't pull away unless you add the belly strap sewn to it like the vestguy style on the gun side.
http://www.thevestguy.com/product/10...0-sg-tac-sling
http://www.thevestguy.com/product_im...MULTICAMO1.jpg
It looks like TVG does the gun side with just velcro where stitch/SG does it with a buckle and the loop that clips to the harness itself is floating on the gun side loop so it has room to slide around a bit. I've seen one actual p90 sling that is TVG's style but sewn instead of velcroed.
In this case TVG's buckle is anchored at the thumb side so if you do something like that you'll need an extension so it will hang lower to make room for your stock.