Seaside feels a lot like desktop development because of its fundamental use of continuations, which lets you feel a lot like you have a session like you do on the desktop. I think Seaside's an amazing experiment and worth wrapping one's brain around, but the web is fundamentally request/response and you'll have a lot more success with a web framework operating in this paradigm -- which, yeah, is going to be unfamiliar to you as a desktop coder.
Whatever tool you choose: at least you didn't choose PHP.
Wait, that's not what I meant to say. (Well, it is, but --)
Whatever tool you choose: welcome. Nice to have you.