I'm looking to run black 18" interceptor steelies for the winter on my 2012 SHO.  First, if I ever upgrade to larger 2013+ brakes will the 18" wheels still work?  Second, do I need different lugnuts than I have with the factory 20" alloy wheels?  Third, any idea how much they weigh compared to the stock 20" wheels?

The 18" PI wheels will work with the larger brakes. You need to buy the PI lugnuts as well because they hold the center cap on and have the groove in them that accepts the ring from the center cap....No idea about the weight but I used them for a winter setup and loved it.

OK:

71.5 LBS with 18" Goodyear Eagle RS-A Police P245/55R18 (no lugs or center cap)

Yea that hurt my Back

71.5 lbs also for the Fire Hawk PVS wheel and tire combo. I haven't seen snow with mine yet, but a lot of the cops here run those tires and don't seem to have problems.

I'm on the fence here.  I can get 18" alloys or 18" steelies for the same price to run over the winter.  Either way I'm running all seasons.  What would you run for a wheel and why?

If it was me, for Winter Wheels, I would run the PI steelies. They have more weight than alloys, and more structural integrity most likely. The extra weight would likely mean slightly better traction. If it is a strictly winter setup, I would also recommend getting a winter tire, rather than all seasons.