I started out on a Slingshot Fuel...I had no problems learning with this kite, and now i'm really glad i got the Fuel over the Diesel...
It just depends how quickly you want (or can) progress?
Ask yourself what kind of tricks, or style of riding do you prefer the most. If your wanting to do the latest wakestyle tricks and need stable kite with a more solid pull, your better off with a medium/ low aspect. If you want to get more power, higher jumps and longer hangtime go for a high aspect style kite.
You say you've had some experience, so i'll assume you can fly a kite pretty well. You cant go wrong with the boxer, diesel or vegas, However you will see a few differences between these and the Torch or Fuel.
- You will get more power, hangtime, higher jumps and a faster turning kite from a Fuel, but the Diesel will handle wind gusts slightly better, turn a bit slower & also comes with ATL (all terrain launch)
- You will get more power, hangtime, higher jumps and a faster turning kite from a Torch, But a Boxer will be more stable in gusts, have more depower and be easier to water relaunch.
So if your up to the point where you can confidently fly a kite overpowered, go upwind, jump and have no problems water relaunching
and you want to start getting longer hangtime in the air, and more power out of the kite, go for the higher aspect kites.
Some pro's today still use the Boxer - its just a good all round kite. You can learn on it easy and you'll never really grow out of it....However, one day you might want to go higher and be in the air longer and have a bit more power, for that you will need a kite like the Torch or Fuel.
Hope that helps a bit.