It's not uncommon to only offer sub-specialty experiences to visiting students, since the sub-I is often filled during application season with students from the actual school; visiting students generally get whatever electives are left over.
I think the answer to this question depends on why you're doing the away. If you're really interested in a specific program for whatever reason, I'd try and get a sub-I if possible, but if they only have subspecialties available it's no big deal--take as close to an intern's workload as possible and find a way to impress them anyway 🙂 If you're trying to get a perspective from a hospital in a different environment than your home school (ie academic vs community, big city vs rural), I'd try and find a program that specifically lets you do a sub-I. If you're trying to get a letter (not that I would recommend doing an away to get one for IM since it's not considered a requirement, but whatever), I'd also lean towards doing a sub-I.