I am pretty sure is weaker. It is like saying I am sure--but with a slight chance that something else could be true. It is like saying I am 98% sure. Whereas I 'm sure is 100%.

I’m pretty sure is like 90% sure, where l’m sure is 100% sure.

It can be either. If you use it on its own, it can mean you're not completely sure. However, if someone questions whether you are right, "I'm pretty sure" can be used to emphasise that you ARE sure. Hope that helps.

weaker. when you say "pretty" in that context, it means you are mostly sure, but when you say "im sure" it means you are absolutely sure.