Is there any reason one should prefer one over the other?
I've always used shortening (probably because that's what my mom always did), but my fiancee has always used butter.
There's no significant difference I can think of (for this application), but I thought I'd ask.
Best Answer
Both will do the job of greasing a pan but there are some differences. One of the big differences is that butter will add a very desirable flavor to whatever you are cooking—which especially complements sweet baked goods.
Shortening is pure fat whereas butter is only about 80% fat by weight. Butter may bring additional flavor to your recipe (even by greasing the pan) but it also adds some water which may not be wanted.
One final difference is that butter burns/browns more easily than shortening.