If it's the same phone I'm thinking of (and I don't think there's many G1 phones around), then avoid this thing like the plague. I saw it in one of the authorized dealer shops, and picked it up... Within two seconds I knew it was a piece of crap.
The software I have no bearing on, but the actual phone was awful. While it does open up into a keyboard, it didn't slide, it clicked up then snapped out, making it feel like a very cheap toy, one that could easily break or stop working all together.
Between the G1 (once again, assuming it's the same model) and the iPhone, go iPhone. Above either though I suggest Blackberry, if you want a smart phone.