I do like cool surplus guns that shoot reasonably affordable ammo, so the 7.62x39 and 9mm both seem like good choices (already shoot 9mm in handguns). This gun would be primarily for range fun (possible 3-gun, maybe someday) and could be used in home defense if things get ugly.
How I see it so far without having handled either one, assuming gun prices are roughly equivalent for guns in similar condition, which seems reasonable:
SKS advantages - very easy to find, large support online, widely available and affordable ammo, plenty of power, lighter (maybe a con?), bayonet

Suomi advantages - very unique and cool, high build quality (from what I hear), slightly cheaper ammo, larger capacity (especially with the drum), lower over penetration risk for defensive purposes
The disadvantages are basically the opposite, so no need to list (unless there is a specific issue that I don't know about).
Anyone have any opinions on either or both? I'm not in a rush, so I have no problem taking the rest of the year to think about it.