Windyway,
I'm a slingshot dealer, but will try to give as unbiased of an answer as possible. I get to test and ride a lot of kites (Cabrinha, Slingshot, Epic, Wainman, etc) and I encourage my local and other riders give input and of all the kites we ride. The Rally has quickly become our/my favorite wave riding kite that also does a lot of other things really well.
The RPM's are awesome kites and do sit a little further back in the wind window which helps with drift, but surprisingly, the Rally drifts just as well. Usually, if a kite sits forward, it might not drift as well. Not so with the Rally. I/we will ride super hard straight towards it and it won't back stall. I HATE backstall and won't touch a kite in waves if it does this.
The RPM's are quick turning. The Rally's are fast turning. If you prefer fast turning, the Rally won't disappoint.
The RPM's are good at relaunch unless underpowered. The Rally's are great to relaunch.
The RPM's are better an unhooking...if that matters, but the Rally is actually good also at unhooking.
The Rally has a little more range.
One of my better friends (Jens) is also on here and he rides RPM's and when we were in Hatteras this fall he would ask for a Rally if it was available when we had a wave day.
Bob
www.kiteridersllc.com