Well I can only speak to the limited one I've seen but the 550 Pro's that I've recently gotten several of have amazingly thick aluminium decks, it must be nearly 2cm thick in certain spots where they've done this to reinforce them e.g around the very bottom on the outsides.
On top of this they've got these reinforcing ribs all over the place - it's really incredibly impressive and they must be streets ahead of anything else out there today.
It seems that they were built to a standard - then what they cost was figured out later, whereas today they build to a price point - deciding that the mower needs to have a manufacturing cost of $xxx and then figuring out how to cut corners and manage to make for this. Completely different approaches and suffice to say it shows.
