You'll regret buying the 3G and being stuck with it (if you're not an impulsive early-upgrader, that is) watching it become more and more obsolete and knowing all along you could have spent a little extra and got something that would have lasted you much longer.
I've also made the switch from a 3GS to a 3G and the difference was night and day. Not only is the speed important to me, but if I follow a link from a website to view something else on another web page then come back and find my original page gone, I'm not happy. Especially if I'm reading a longer article and have to find my place again. With the 3GS I've had as many as five different windows open at once. With the 3G you're lucky to get two.
And lastly (but certainly not least....ly) that chrome lettering on the back is so much nicer than the faded dingy looking grey on the 3G.
I didn't actually realise you could have multiple browser windows open at the same time (I guess the sidekick pales in comparison to the iPhone immeasurably), which seems like a little thing, but because I'm prone to using the internet on my phone a lot I'm sure it'll make a hell of a difference to me. 