Firstly, thanks to Craig and Andrew for the advice!
I did look at most of the applications that can deal with the binary format of the GT-31 device. For many reasons I currently prefer gpsbabel for my conversion runs.
What I was trying to find out are two things:
1. Do I log in binary format or in NMEA? It is either-or with the device, and I wondered which format is the "better" one.
2. Whatever format I use, what can gpsbabel do with it.
As to question one: I was under the impression that SIRF Binary is like a digital camera's "RAW" file format, and can be converted into any other, "less capable" format later on, such as NMEA. After reading through some SIRF documentation I learned that this is not the case. A SIRF device can operate either in "binary" or in "NMEA" mode, and apparently performs different calculations from the raw satellite data into the recorded protocol format.
In "binary" mode the precision signals are converted into a number of statistical values, such as "EHPE" (estimated horizontal position error). These values cannot be "turned back" into the NMEA protocol's HDOP, VDOP and PDOP (the HDOP is retained in SIRF binary, though; and GPSResults seems to be able to reconstruct a VDOP anyway). Gpsbabel's filter can only deal with the NMEA values, so if I want to clean up track data with gpsbabel I have to use NMEA.
--> At the moment, I will record in NMEA format with the device.
As to question two, I wanted to use gpsbabel for two things:
1. Clean up track logs, join/split them (join files, split them into days), and re-save.
2. Convert to Garmin and/or GPX format.
For these tasks, gpsbabel performs well, with some limitations:
1. It loses precision in the conversion run (e.g. from 00838.2842 to 00838.284)
2. It drops all satellite information (satellites visible in the $GPGSA record, all of the $GPGSV records)
This means I have to archive my original data as well as the converted ones.
Best regards, Martin