Hey guys, ive learnt kitesurfing with a Slingshot RPM, I am considering changing to Rally, does anybody know how they are different, and how one should be better than the other?

You did things a bit back to front
generally the rpm is a step up from the rally, if you haven't progressed a huge amount or are early in your kiting you'll find the rally is an easier kite, very much more sheet and go and park and ride, than the rpm which prefers to be actively flown, if your struggling with the rpm I'd say you possibly need to step back for a bit until you've got a few hrs under the belt, imho though your kite skills will improve quicker on a kite that needs to be flown

have flown diesels. fuels , octane , rallys. rpm over the years imho rally has a lot better depower , launch and upwind height using same board in similar conditions but rpm alot better loft . tend to fly rally on leach a lot more to power up and work on flow with rpm( you have to fly the rpm a lot more) depends on where you want to use mostly rpm more mid/flat aspect freestyle rpm better dial down depower.
rpm like the fuels develop more power( although a flatter arc) but the trade off is less wind range and depower throw.\) . i have gone to bandit 10 this year and am liking the kite but hating the bar. found compstick control bar from my rally 12m works well with bandit

I think Canucy is on the mark, the Rally is a better beginner/intermediate kite that will preform well as you progress, big wind range, heaps of grunt, and easy to fly.
The wave sst is another good slingshot kite to consider too depending on your future needs. not sure where you are try and get to a Slingshot demo day.
Either way you will get a great kite.

^^^^^ I agree with the above comment, if the rpm isn't the kite for you, and it's not everyone's cup of tea. I would go with the sst.
It was slingshots wave kite but because it was so good at everything else they aren't pushing it as a wave kite anymore. Boosting on the sst is proper good!!!! I would consider this as an alternative to the rally if your after a more performance kite rather then ease of use.
Ether way, your still going to get a sic kite

I'd suggest trying those girls kites slingshot make, FUELS.. Much better all round kite with 18 odd year's of development now. Nothing compares to a load and pop kite like a King of the C kite FUEL. Girls even ride them#notefordapussies
Or Rpm is a weapon... No 1 like Youri Z wins on a Rally??? Strange.. I know, he doesn't eat enough weetbix!

Really depends on what type of kiting you like doing or want to progress in....