Not much in physical looking outside dimensions, the 12si is heavier, about 1lb, the 10si weighs in at about 10 lbs.
The 10si was the first SI (internal regulated )alt. and has the upper adj. bolt thread 5/16x18.
The 12si came later and has higher output capabilities and better cooling to deal with under hood temps on cars of the 80's.
The upper adj. bolt thread is metric-M8x1.25mm.
A 10SI can be had up to 94 amp so I don't see a need to go to a 12SI for where you intend on using it.
Dan
