On personal bias, I like Innes more than Seth. Seth has many, many redeeming qualities about him. He is strong and kind, compassionate, but not without some power in his position with beliefs, thoughts, and actions.
Innes has more of a professional outlook and in a way, is more fit for battlefield expertise and leading a country. He doesn't tend to allow his feelings to affect actions taken in war. He's less likely to become irrational or to be driven to brash action.
As a more logical and calculating person, I associate myself more with Innes and I've always had a bit of a soft spot for him. Eirika would be a good complement for him. They are both excellent men for that form of serious relationship, but with Inness being the professional and rigid man he is, Eirika's soft demeanour is more an asset for him.