For best GPS performance/stability I highly recommend using GPSGate from Franson it makes all the difference.
I have both OCN6 and Iguidance4
The neat category lists things that one can do that other can't that aren't directly related to performance but neat feature.
OCN6:
PRO
customizable interface (pdamill skins)
"eye-candy" graphics
Detailed graphics
Better map view and angled "3D" view
Smart Zoom - I have overhead view when more then 10 miles to next turn. Then 3D view at <10 miles and then it will zoom in close when 200feet from turn
larger POI database 12+million
Can sync your clock with GPS
POI import
Smooth screen update while driving
Preset for Home and Work
CON
NO Text To Speech for streets
No immediate turn info - Let me explain this: Lets say you have to make a left turn then you have to make another turn within a couple hundred Iguidance will show an two icons. In OCN it will you only one turn and it may tell say "turn left then in 200 feet turn right" But, not always
Quirky routing. I've had it take me off a highway onto surface streets to save 0.9 miles. OCN prefers interstates (65mph) over state routes (55mph) and will always try to route you onto an interstate route if possible.
Dedicated SD card
Manuveurs not as detailed - Voice will say: "In more then 2 miles turn left"
NEAT
In 3D view the realistic clouds and stars in the sky. If it's near sunset or sunrise the sun will appear in the correct spot on the screen!
Iguidance:
PRO
Less eye candy
Will tell you what side of street destination is on
Modify hardware button actions to suite
Text to Speech for street names
Routing is more logical
Detailed voice routing - Voice will say "In 2.5 mile turn left onto Anystreet" Where Anystreet is the TTS engine saying street name
CON
Less POI - Although more POIs then previous version they actually removed POIs to add more.
No POI import function
Warning screen on startup - you can disable it by using a shortcut
1 second update of screen while driving
Voice for guidance is different then that for TTS engine that says street names
Preset for Home location only
NEAT
Info screen for town you're in (name of town, Even/Odd house numbers on left/right), etc
Weather info based on GPS location (requires internet connection)
Almanac screen showing sunrise/sunset time and moon info
That's been my view of them. Right now I've been using OCN more then Iguidance because I like the look better. But the quirky routing can be an issue.
OnCourse Navigator - Discussion
Iguidance has the TTS for street names as one of the big pluses and it doesn't require a dedicated SD card.
However OCN has more options and POIs and just looks really nice.