As an Ortofon dealer, we have enjoyed the opportunity extensively tested most of the SPU cartridges. Lencotweaker nailed it - the SPU A/G/N cartridges all sound different. There is actually a lot of technology in the headshells for the G series, which accounts for the cost difference.
Also, the G and N series cartridges with similar names do exactly correspond - the G series has some newer technology and designs.
In general, I would say that, like for like, the G series sounds more "SPU-ish" than the Ns, the latter which I would categorize as more neutral and less full and lush sounding.
The key to selecting an SPU cartridge is NOT to choose based on price, but to evaluate your LPs by genre and style (early 50s mono jazz vs. early 80s stereo hard rock and select based on the most appropriate SPU stylus type.
If you were looking for one cartridge to start off with, and if you have the right tonearm, I would recommend the SPU Classic GM E MkII as a great entry into the SPU system, for a reasonable price.