I've used both - btw fully functional demos are available on their respective websites. Heres my opinion:
Interface: Ableton Live has a cleaner look and is more intuitive to use.
Virtual Instruments: Fruity Loops has more instruments, but Ableton comes with gigs of professional sampled sounds eg. real acoustic piano, jazz guitar, violins etc.
Sampling: In Ableton you can chop up a sample and add the individual slices to Impulse - the built in drum machine - and then trigger the parts using a midi controller, very similar to Fruity Loops.
Effects: You get your usual EQs, compressors, and reverbs in both programs - however in Live the way you set up the effects is much more fun, using a rack like system.
Workflow: This is where Ableton Live blows away the competition and what its famous for. It quite a unique setup where samples, loops, midi data are triggered from a set of stacked vertical cells, and this then recorded (real time if you want) onto the traditional horizontal sequencing grid.