I've owned both and by far I'd say the Benelli.
My Saiga is a Short-Barreled Shotgun and it's a fantastic range toy to turn heads and clear the skeet range, but for practical utility it's lacking. You can't store it with an empty chamber and a loaded magazine, it's awkward to reload (though how often you'll reload with a 20-rd drum is another question), for it to reach it's potential you have to extensively modify it, the safety is awkward and a whole host of other problems.
On the other hand the Benelli is like a sports car from the factory and is ready to go right out of the box. It'll feed anything, it feels good in the hand, it looks good and it's just a friggin' nice shotgun.
Get the Saiga if you want a gee-wiz toy to wow your friends, but if you're serious about a shotgun, get a Benelli.