Yea the 9 arc second TAWS function is nice to have but not essential (IIRC 9 arc second is the same as the 496) unless you use it to monitor an IFR approach. The main reason we went for the 550 as opposed to the 500 was that I couldn't find anywhere about interfacing the ZAON PCAS to the 500 whereas I found some articles about interfacing it to the 550. I expect you can i/f to the 500 but I didn't want to take the chance that I couldn't.
Our 550 also came with a yoke mount and dashboard mount and we bought the bare wire kit to wire it into various places (audio panel / zaon / power).
One neat feature of the 550 is the way it clips into devices such as the yoke mount / bare wire kit. The 496 you have to plug the cable in, in the Aera you just clip it in and all the connections are made, bit like a TomTom. As it is slim you could mount the "bare wire" kit connector on the panel somewhere and then just clip the GPS in and out as nescessary. Useful if you use it in one aeroplane more often.