I'm usually more comfortable on ice with no traction aiding devices at all, but thats just me. But since you want to upgrade how about an ARB air locker. That way its either 100% locked, or none at all. Its just expensive to do.
I don't think a spool matters on how fast you drive since a bunch of drag cars have them. I'd just worry about taking corners on pavement with one.
'73 K5
Chevy good...Ford bad