I think you'd find that if you had a smartphone you'd use it a lot more than you think - especially if you're catching transit. Streaming radio and video can be huge for killing time.
I use mine all the time, but don't use all that much data at all because I generally don't stream anything (well, maybe 5 min a day on average). A lot of that is because I don't catch the bus or anything like that, and also because I already have two ipods I use to listen to music. Also (and this is a huge bitch) my Omnia doesn't have a 3.5mm headphone jack. It has a Blade jack for everything and an adapter I can use for headphones on that. Not a huge deal, but something which just makes it a little more of a bitch to use when I could just ipod most of my shit. DivX support is awesome though, and if I ever have 30 min or so to kill while waiting, I'll hook the sound up to my car speakers and watch perfect quality movies.
All that aside, I'd have a hard time going back because I've gotten so used to the benefits of a smartphone like syncing scheduling software, MS office support, and...well, a ton of other shit.
Yeah, after having a BB Pearl, Motorola Qm (piece of shit), and then BB Curve, I just can't see myself ever going back to a regular cell phone. Even one of those things like the Samsung Alias wouldn't be enough. I'm hoping to get a Droid soon or a Nexus One when it supports Verizon.
Both those look sweet. The thing that sucks about smartphones is that everytime a new one comes out I pretty much want it. If you're looking at a Nexus One, however, I'd see if you could play around with one first. A lot of people have trouble switching to a touchscreen only phone - typing can be a bitch at first.