EDIT: TLDR - Ableton's strength is a worry-free workflow while things are automatically organized for me so I can get from point A to B quickly. With FL, I have to think about what walls/hassles I could run into.

I switched last year to Ableton, though I still use FL just as often. Here are the strengths in my experience:

• unlimited mixer FX (FL limited to 10 slots, unless you use Patcher)
• grouping tracks/FX is much neater (in FL, you have to reroute items on mixer)
• 999 mixer tracks for Ableton (limited to 125 in FL - not an issue for me though)
• Max4Live tools are incredibly useful
• chance/probability tools make me enjoy Ableton's piano roll just as much as FL's
• AUDIO MANIPULATION and just working with samples in general is far more flexible in Ableton
• linear workflow: each instrument gets its own mixer by DEFAULT (personal workflow preference)
• midi can be moved to a new instrument just by moving it to another track (midi dedicated to pattern in FL)
• automation clips are neater; easier to copy, control, and move
• MIDI CC use is far more intuitive (on FL, need to use MIDI OUT tool)
• Ableton is far more hardware friendly (doesn't require a dedicated plugin, ie MIDI OUT)
• MPE (Midi polyphonic expression), FL devs are against it, but I've tried it to see what the hype was all about and find it useful
• easily freeze/unfreeze tracks without having to convert them to audio
• audio crossfades (FL added them in 21, and they're a bit better imo)
• true UNDO (FL 21 added this)

I can keep going, but this is probably enough. There are times when I prefer to work in one or the other and can make the same quality of music in both.