I say get Live as it's far more immediately accessable. If you get Vanguard with it, (a cheap but lovely VST synth) you'll be able to use your MIDI keyboard to create fat sounding trance leads, arpeggios and big pads and stuff.

Get a load of sample CDs for loops, percussion and effects, and you're away!

Get Reason later if you want a very creative tool for intricate sound design and a fully featured sampler. It's hard - very hard - to get release quality out of Reason... unless you use the NNXT sampler with good samples, and pump the result into a Rewire host - like, for example, Live.

Reason used as a drum machine is incredible! Reason as a synth is interesting, but not fat 'out of the box'. You'll need to learn a lot about synthesis and sound design to get the best out of it, whereas a lot of VST synths - which you can't run in Reason - sound big immediately.