IMO, the shell of the Plat 6 was a severe step backwards from the Plat 5, in terms of sturdiness. The Plat 7 is better.

However, being a carry-on, that probably won't be your major concern.

The nylon on the Plats is a better quality than on the Crews. Whether or not you think it's worth the $ difference is up to you.

Personally, I have the Plat line on my check-in pieces and have the Crew for my carry-on.