I recently learned about both of them meaning "I intend/plan to..." but I don't know the difference between the two. Can someone explain?
Do you understand the difference between an intention and a plan?
つもり is just your intention. You don't necessarily have a plan. Stating an intention doesn't mean you need to go through with it.
よてい is a plan. You've made a commitment to do something, with just yourself, with someone else, either way. Not following through on a plan suggests you're perhaps not someone who can be counted on.
On a scale of definiteness, it seems to me that
is the least definite;
sounds a bit more definite;