Hi,
I was the chap with self destructing SD holder!
My opinion is: If you can afford the 595, go for it.
I am glad I switched to the 595. yes it is more expensive, but there are other considerations.
The screen is significantly larger than the 396. I have (now a reserve GPS) a BMW Nav IV and the screen size is similar to the 396. However I (and my wife) felt that the display was not as clear on the 396 as it was with the Nav IV. The road lines are narrower than on the old device.
The sd card holder on the 595 is in my opinion better than the 396.
However the Bluetooth connection on the 595 is not as good as the 396. I no longer use the phone in conjunction with the intercom, as it would keep dropping. The Bluetooth to the intercom drops periodically, but less when there is only one connection. Of course for all fairness, my intercom is very old and that might have an impact on the stability of the connection. The 396 worked perfectly with the my Bluetooth connection.
Both are supplied without a case, which I think is wrong of Garmin especially as they will be stored in bike glove compartments, pockets, etc. The possibility of the screen being scratched is quite high. I bought from my local BMW dealer a case for the NAV VI which fits the 595. Yes it was more expensive, but the Garmin one was only available after 3 plus weeks. I had tried a generic, and it was neoprene, not very safe against sharp objects!
Certainly the 595 having the audio out cable, makes having a wired intercom that much easier. The 396 does not have this option, so any connection to a wired intercom has to be via Bluetooth.
The case against the 595 is if you buy the travel edition then it is not easy to have your maps on the computer. It Is not possible to install the maps on the computer. There are ways around this, but it is not as easy has having the map already installed.
The other point to consider is mount size. The 595 mount is massive, I am limited in how far I adjust the 595 down to avoid the glare from the sun - this was not the case on the 396.
With regards to upgrading, if the 550 is showing signs of age, I certainly would upgrade. I upgraded my Nav IV a year or two back, and the updates are now infrequent, they do not fit on the internal memory. Plus it has difficulty in finding the satellites, whereas both the 396 & 595 are virtually instant connection.
Best regards
Sleuth