Learn English – “Supposed” versus “expected”

To suppose something means:

1. to assume (something), as for the sake of argument or as part of a proposition or theory: Suppose the distance to be one mile.
2. to consider (something) as a possibility suggested or an idea or plan proposed: Suppose we wait until tomorrow.
3. to believe or assume as true; take for granted: It is supposed that his death was an accident.
4. to think or hold as an opinion: What do you suppose he will do?

To expect something means:

1. to look forward to; regard as likely to happen; anticipate the occurrence or the coming of: I expect to read it. I expect him later. She expects that they will come.
2. to look for with reason or justification: We expect obedience.
3. Informal. to suppose or surmise; guess: I expect that you are tired from the trip.

Suppose implies a guess (not what you necessarily think will happen), while expect means that you are pretty sure it will/should happen.

So expect is better in your example:

I often use one when the other is expected.

However, even that implies that a certain person is expecting one or the other, rather than that one or the other would actually fit better. I would say:

I often use one when the other would fit better.