Hey Tekno - Ive flown all of the above. Heres my 2c's worth.
I spent a short time on a low aspect beginner kite (like the diesel) before swapping to 4 line FUELS. OK back then I guess but in todays world a bad move. As Greenpat says FUELS are high performance 'C' kites which are really suited to higher performance riders who require, and can manage, the raw power. They are way less forgiving & will slow the learning curve for newbys.
The diesel was a low aspect (ie fatter; search it if you need to) kite but with none of the huge depower range of the newer bridled flat kites. Low aspect kites are far more stable as they sit back in the window more solidly and are therefore slower & easier to control. Before the high depower bows and hybrids appeared this was about the only means of tuning down a kites performance to make learning a bit easier.
All that changed in 06 with the bridled (SLE) 'bows' and the soon to follow hybrids. These really are the kites to learn on (for 95%) if you want safe, controllable gear which will get you up and running way quicker. The Turbo Diesel is SlingShots entry level SLE (supported leading edge) styled kite. Way superior for entry level than any of the 4 or 5 line 'C' kites - inc the FUELS or the more demanding SLE hybrids like the REV.
An older Diesel will get you there for a lot less money than a Turbo Diesel but it wont be as much fun or as quick. An 06TD & esp the later TD2 would be the WTG if you can afford the extra cash. They will keep you smiling well up into immediate level and; some would tell you; way beyond that for surfing.
All purely my opinion - FWIW
Hope it helps.
ps - esp this year - there a lots of good kites and good kite companies out there to choose from. For mine Slingy have always given unbeatable quality & back-up service. And remember bar set-up is important too.
