I'd go with the 12" sub, but the sub has to be appropriate for the enclosure you run. 8's or 10's being "faster", or "punchier" or "better for metal" is the oldest sham in the book....it's a 25 year old theory that has been proven incorrect for years. Back in the 90's this may have been true, but only due to the exceptionally large enclosures bigger subs used to run or be modeled after...things have changed, air space requirements have changed, technology has improved. A well modeled 12" or 15" will be just as "quick" as any 8" on the market. But it comes down to which sub, and the enclosure specifications. All things being equal, a 12" will sound better, fuller, dig deeper, and be more comfortable going super low than any 10 or 8....and a 12" will not need to be pushed as hard for the same output as a 10"....the sub/enclosure combo is key
I like Arc Audio
Image Dynamics
Audio Frog
Dynaudio
Skar
Jl Audio