I suspect there aren't any differences: I imagine the replyto was added to <cfmail> for expediency, given it'll be a reasonably common param to wish to set.
The quickest way to check fro yourself is to send two emails:
- with the replyto address in the <cfmail> tag;
- with the replyto address as a param.
Inspect the source of the email messages when they arrive and observe there's no difference.