This message I will reply to if/when I'm around. I know exactly what I'm getting into. "Hi" could be I have a quick two second thing, or I'm about to dump 6 pages on you and expect a reply in a few minutes. I'll reply when I'm prepared to deal with the latter. Maybe.
Yes exactly. I'm agreeing that this is the right alternative and will get a response from me much faster than the empty "Hi". "Hi" <brief description of why I'm reaching out> is always superior.