Doesn't your sg have P90s? Mine does, and I've never owned one with humbuckers...how do you think they compare? I guess it's apples and oranges what with different manufacturers and stuff, but in general...? I'm guessing a meatier, more mid-rangey sound?
My SG is an Epi and it never had P-90's. I replaced the stock junk Epi bridge no-name humbucker with a Gibson 57 Classic and Gibson pots and wiring. It helped a ton. I never use the neck pickup so I left it alone. I like the lower output humbuckers like the Burstbuckers and 57 Classics. They have a dirty vintage tonality that hotter pickups can't touch.
I had a P-90 SG a long time ago, but it got stolen. I might put one in the SG now that I have the LP doing the humbucker heavy lifting. I like single coils and they suit my kind of music very well. Before getting the LP I'd typically do the SG with the humbucker on one side and my Strat with singles on the other. That's a good combo for a varied but big kind of collective rhythm sound. Single coils through a crunchy tube amp are the sound of vintage punk. I like to mix in some humbucker tone though.