Copper is a nicer product. Wire sizes will be smaller than Aluminum, and connections tend to age better. If can afford it, copper would be a better choice. I'll just throw this out there, but if you are putting in a 100 A feed, wouldn't you feel better putting in in rigid? Anytime I do a garage, I put it in GRC and I have yet to have a call back for a shovel through the raceway. Sure, it's more up front, but you'll NEVER have to worry about it again. (Failing some unforseen catastrophe).