In general, for verb that represent actual physical actions, there's a degree of semantic difference between the present progressive and the present simple.
I run to school.
This is a statement of general truth; the person saying it is asserting that they are in the habit of getting to school by running.
I'm running to school.
This is a more precise statement; either they are saying that right now they are running to school, or they are using it as a progressive form of the futurate, as in "I can't give you a lift in tomorrow, I'm running to school".
(There are some other possibilities for both, such as the historical present, but they are not worth concerning ourselves with here.)
However, when it's a stative verb or a verb of mental action, the difference is less clear. In some cases, it will become relevant, but most of the time there's no clear and definite semantic difference between the two. For example,
I hope it doesn't rain.
I'm hoping it doesn't rain.
No clear difference whatsoever. It's a matter of personal style, sometimes in a specific case there will be a difference in nuance, and it's likely that which is used more often is heavily dependent on dialect. In the specific case you give, I would read a small difference in nuance; I am assuming suggests to me that the person has just reached the conclusion and it is still tentative, inviting contradiction, while I assume would mean they feel that the assumption is safer. However, my reading it that way might be to do with the dialects that I've grown up with.