Duchene's a faster skater, IMO, particularly top end speed. Bennett's got great exceleration and speed, mind you. But Duchene can outright fly. Bennett's more physical/feisty. I'd also say Bennett's got a better shot/scoring touch (at the same age).
For those that remember his Kitchener days, I'd say if anything, Bennett's more like a more explosive skating, better shooting/stickhandling Mike Richards. Richards has the edge in fighting ability and consistent hitting game, though.
Defensively, Bennett's already dependable. He's also on the Fronts' PK unit (along with linemate Ikonen).
I used to like the Gilmour comparison, but the more I watch Bennett, the more he looks like a better goal scorer than Gilmour, but not quite the playmaker Gilmour was. Similar "dirty/gritty" style of two-way play, though.