Well, I think any Arduino will bring you a step forward, but I wouldn't suggest a Pro, or Fio or so that need an external USB-Serial(TTL) converter to program it for playing around.
An Duemilanove, Mega, Uno should be fine. I bought a Mega 1280 from DFRobot first and I am very happy with it. It (with some additions) is now running as a RS232 Data logger on our Cloup Droplet Probe.
I bought it on ebay from a chinese, but he has a logistics center/partner that sends it from Germany. A bit more expensive than China, but not much and very fast. I bought several times there.