I recently purchased a monitor calibration package and needed to make the same decision you are. The three products I compared were ColorVision Spyder2 (~$175), GretagMcbeth Eye-One Display 2 (~$250), and Monaco Optix XR (~$225). The three products seem closely matched. This review helped me eliminate the Monaco Optix product:
http://www.drycreekphoto.com/Learn/monitor...ation_tools.htm
I ended up buying the GretagMcbeth product because 1) I liked the software interface better than ColorVision's 2) It includes an ambient light measuring capability to ensure the my work environment is optimal.
My monitor looks a bit subdued now that it is calibrated. The photos I posted on Monday here on wetpixel were posted from home on a screen that was not calibrated. When I came in to the office and viewed my post on another monitor was very surprised how dark the shots looked. I immediately lightened, then reuploaded the shots. I received and installed my calibrator later that day and hope I will not post more embarrassing photos in the future.
I might also add that my initial motivation for screen calibration was very poor color rendition on a Shutterfly bound photo book I created to give as a Christmas gift. It's interesting how, post calibration, the photos in the book match the same photos displayed on the screen. :-)
I guess it's now time to readjust the photos and generate another $50 book.