It depends on the context that you are using it in. If you are saying it to someone sarcastically, such as if you haven't seen them in a while, then you would use 'suppose.' If you would use assume, it would make the statement more definite, as if you were to carry on with things as if they had died. It might be different in other contexts, but generally, that is how the two are different.

what about "guess" ? I think.. it's better