Rob,
The plan is for the Port 30003 output is for it to be compatible with the SBS-1 add-ons that use the data out from the SBS-1. However, as the add-on programs have been written by different people, they use different bits of information.
The port 30003 output from RadarBox will almost certainly change at some point, but not immediately. Depending on how difficult it would be for you to change your program, it would be really interesting to see what you have produced.
Maybe go with a beta, but be prepared to change it?
The port 7879 output is an AirNav standard, so this is unlikely to change. All the messages do contain lat/lon, but not only lat/lon, there is all the other relevant information too.
Here are 3 lines of data I've captured from port 7879 - although the data comes out as one long stream rather than separated like this.
<MODESMESSAGE><DATETIME>20080321142534</DATETIME><MODES>4006E6</MODES><CALLSIGN>CFE96C</CALLSIGN><ALTITUDE>28025</ALTITUDE><GROUNDSPEED>341</GROUNDSPEED><TRACK>330</TRACK><VRATE>0</VRATE><LATITUDE>52.9320</LATITUDE><LONGITUDE>-1.2899</LONGITUDE></MODESMESSAGE>
<MODESMESSAGE><DATETIME>20080321142534</DATETIME><MODES>4CA555</MODES><CALLSIGN>BCY500F</CALLSIGN><ALTITUDE>32000</ALTITUDE><GROUNDSPEED>336</GROUNDSPEED><TRACK>308</TRACK><VRATE>0</VRATE><LATITUDE>52.2806</LATITUDE><LONGITUDE>-3.8336</LONGITUDE></MODESMESSAGE>
<MODESMESSAGE><DATETIME>20080321142538</DATETIME><MODES>400B26</MODES><CALLSIGN>EXS897Q</CALLSIGN><ALTITUDE>07975</ALTITUDE><GROUNDSPEED>249</GROUNDSPEED><TRACK>079</TRACK><VRATE>3008</VRATE><LATITUDE>53.3793</LATITUDE><LONGITUDE>-2.3146</LONGITUDE></MODESMESSAGE>
Hope this helps
Allocator