Todd, If you are significantly closer to a Coast Guard ground reference station than you are to a WAAS ground reference station then the coast guard correction will probably be better than the WAAS correction. You can find location of WAAS ground reference stations by googling "WAAS reference stations" and browsing the links that google finds.  All ground reference stations use dual frequency GPS receivers that are capable of measuring ionospheric delay at their location. WAAS transmits corrections for ionospheric delays that are interpolated from that data for locations between WAAS ground reference stations. Those interpolated ionospheric delay corrections are guesses because the interpolation model does not account for local ionospheric anomalies, they use a 'smoothed guess'. Here is a link to a picture that illustrates the dynamic behavior of 'smoothed' ionospheric delay (total vertical electron count is a measure of ionospheric delay).

http://gpsinformation.net/exe/iono-day.gif

In the real world, ionospheric delays are much more dynamic and 'bumpier' than those shown in that picture. That's why it's best to use differential GPS corrections that come from ground reference stations that are nearest your location. The ideal is RTK, where you have your own ground reference station, but RTK has it's own set of problems also.