Very common question.  Both cost about the same price.  So which do you choose?  Here are the pros and cons:

Superwedge - Pros:    Polar alignment is more accurate than altaz; less corrections needed during guiding, especially along declination axis.  Permits piggyback photography. Permits use of PEC or Smartdrive on scope for more accurate tracking.  More precision in GOTO performance.   Cons: Heavy; harder to transport.  Increases amount of flexure between tripod and scope; more sensitive to vibrations and wind (note: an extra hundred bucks for the Mettler and Milborn wedges, or two-fifty for the Mitty Superior wedge, provides better solutions to this problem).  Takes much more time to setup in the field.

Field Derotator - Pros:   Light weight; very transportable.   Easy to setup in the field; don't need to go through the lengthy process of polar alignment. Cons: Requires power in the field.  Inability to do piggyback photography.  Adds weight to the visual back.  Extension to visual back may cause scope not to clear the fork; you lose a large part of the sky. Vignetting increases due to the extra extension.  GOTO precision suffers.  Objects directly overhead tend to spin too quickly for use; you lose the upper 20° of sky (the best part).  You must make more guiding corrections, especially in declination.  Inability to use PEC or Smartdrive.  Cannot use a separate guidescope; must use an off-axis guider (granted, an OAG is preferred for SCTs anyway, but guidescopes are still excellent for shorter exposures).

Simply put, the wedge is much more versatile at the expense of portability.  The ability to do piggyback photography is very big for me, as it should be for you as well.   The flexibility to use a guidescope on occasion is rather nice too, especially when guiding with the eFinder attachment of the SBIG STV.  But the person who travels to their dark site and primarily does prime focus photography will appreciate the ease of setup and portability of the field derotator.   But unless this describes you, go for the wedge, especially if you are in the beginning stages of the hobby.  

When you do get your wedge, I would suggest doing a modification of the hardware components to make setup more smooth. Try the following links for how-to information:

http://easyweb.easynet.co.uk/~chrish/lx_swmod.htm
http://www.valleytranscription.com/astro/linkframeset.html
http://www.bas-astro.com/projects.cfm (PDF file)
http://65.211.227.74/greg/html/superwedge_azimuth_adjuster.html

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