Aera 500 = 495
Aera 550 = 496
I think?
Why did they have to give it just a silly name?
I suspect that the real cost difference is in the hardware. The 495 family are only really used for small volume aviation and marine markets, whilst the Aera is basically a mass-market Nuvi unit, which they can churn out for a few tens of pounds, so you're only really paying for the software.
With the screen brightness as low as keeps it readable, I'm getting about 2.5 hours life out of the Aera without recharging. When I bought mine, I did also order a spare battery, which has turned out to be a good idea. So far, I'm very happy with my Aera 500 - the higher resolution ground map of the 550 would be pretty, but I doubt would give me any extra real benefits.
There are a few things I'd like to change on it, but not many. I'd prefer the audio warnings to be A LOT louder, given I'm normally wearing headphones when I'm using it. Airfield diagrams would be nice (I did have those on my old Pocket Flymap, but that had so many other deficiencies that I stopped using it), and it's irritating having to take the back cover off to plug it into my laptop to charge or transfer data. That said, it's still by far the best aviation portable GPS I've used. I am still getting used to it however, so may well have a few more comments in a few months.
G