The Locosys GT-31 is being developed with specific features for GPS Speedsailing (www.gps-speedsurfing.com) and for our purpose we need to have the DOP, number of satellites and some other data that relates to the accuracy of the data. This data is present in the Binary format files .sbn (written to the SD card) and .sbr (written to the internal memory). Data can optionally be saved in the NMEA format to the SD card and includes HDOP info.
The programs we use for analysis of our speeds (GPS Action Replay Pro, RealSpeed and GPS-Results - see below for links) are specifically designed to use the HDOP and sat count info to filter the bad data points out. The programs above can take advantage of the more accurate data saved in the Binary formats by directly reading those formats and can also convert that saved data to NMEA. (after filtering if you wish :-)
Our specific interest is very accurate speed measurement so we use the Speed Over Ground data derived by Doppler analysis of the Sat signal rather than the Positional data. By doing this we believe we can get to a speed accuracy of better than .002 knots with a very high degree of probability.
By using the Binary data from the GT-11 (superseded model) and the GT-31 you will also get higher precision in the positional data (to more decimal places without rounding) but the accuracy is of course not necessarily better unless some sort of post processing correction is done. Apparently the GT-11 firmware has been tweaked to produce data in a form that is suitable for post processing to a high level of positional accuracy but I understand the GT-31 data is not as suitable for this. Something to do with the change in GPS chip and firmware and the output from those particular models.
We have also been playing with the MTK 5hz chip to see if we can get the same or better speed accuracy at the higher hz but at the moment the best data seems to be from the GT-31 with it's Sirf3 chip. I believe this is more to do with internal filtering and firmware rather than the potential of the chip itself and development continues.
AFAIK the Bluetooth versions of the GT-11 and GT-31 (BGT-11 and BGT-31) have exactly the same capabilities apart from the addition of Bluetooth communication.
If anyone wants more info in the file formats or specific capabilities of the units just ask and I may be able to point you in the right direction.