Eh, I can't really imagine a romantic Satoshi so I'll give this to Haruka. Although once in a relationship, the boys starts to become the more romantic type than girls so it may depend on the situation. Should we say that they are only dating at the moment, Haruka would be the more romantic one but if they had entered a relationship, I'd say Satoshi would be the romantic one but not too romantic, I mean if we're dealing with his casual personality, he'd show his romantic stuffs his own way.
Anyway, I think it's a good time to bring up this question:
How did you became an advanceshipper?
For me, it started back when 1st say Haruka back in early 2003. When saw her character, I knew she'd be a suitable heroine for Satoshi and their chemistry would be great.