Get 15"!!!!
I was in the SAME situation!
I got the 1 GHz 12" Superdrive model in mid-March, and pretty much disliked it from the start: Screen too small to work effectively, no card slot, doesn't look elegant from the outside since it'as thicker... simply felt more like a silver colored iBook than a professional grade machine.
When the new Powerbooks were announced just a month later, I took the plunge, sold the (almost new) 12", and got the new 1.5 GHz 15" with Superdrive.
I have to say, I could not be happier with the upgrade. To me, the 15" is just infinitely better. The screen size difference is huge in terms of making it easier to work. This machine feels professional.
If you feel buyer's remorse, rather than live with a mchine that doesn't fit your needs perfectly for a few years, I recommend spending the few hundred bucks it'll take to upgrade and get the machine you really want. If it makes you as happy as it made me, it's well worth it.
One thing I have to say,though, is that the 1 GHz to 1.5 GHz speed difference isn't quite as noticeable as I had hoped it would be. but I'm still more than happy with my upgrade.