Hi there. This is an excellent question.
What you can do is to review them side by side and let the visitors decide which one would be a better fit for them.

Hope this helps,

Cheers

Hi - it depends on the affiliate program. Some allow you to promote competitors, some don't. You would need to read their terms and conditions.

Or do you mean, promote more than one product/service from the same affiliate program? That is fine.