Since there seems to be a lot of proG2 replies, I'll counterpoint for G1 (I also have one, never used a G2). Currently the cheapest price for a 4gig G2 appears to be $200 shipped vs $80 shipped (buy.com) for a 4gig G1. Let us assume you have never heard a G2 so while theoretically it may sound better than a G1, if you don't do an A/B comparison, you won't know (both will sound better than Satrad).
How are you going to use it? If in vehicle or at work, size doesn't matter. Conversely using the reverse size argument, the G2 has a smaller display. As for accessories, let's face it, nothing beats the ipod family of accessories so while the G2 has more than the G1, there aren't a lot. That being said, it doesn't look like the dock featured in the included pamphlet will ever come out (for the G1) and probably no new accessories will come out. If you want accessories, get a G2.
If you're thinking of dipping your feet into Slacker, the lower entry point of a G1 will give you a good feel for how you like the service/device and you won't be missing out on any features. If you are patient, Woot occasionally has the 2gig (15 station) G1 for $50(+$5) which would be an even better starting point to determine how much you might enjoy it. Just my $0.02 from a G1 owner. I don't feel like I am missing out on not owning a G2.