Welcome to the forum and hopefully to the s5 family. I have an S5cab as my daily driver so I'll offer some thoughts. There really isn't another 4season convertible out there with the S5s features. For 60ish you get a beautiful all wheel drive sports tourer. Awd. Beautiful in and out. Great top. Don't see tons on the road. The performance is great. You can tour around casually and comfortably or push it and have a real burner. I haven't driven the e350 cab. It's pretty inside and out albeit far more posh than aggressive. The 350 won't perform like the S5. And while the 550 might keep up straight line I doubt it's as much of a performance cab as the S5. Nor does it have the awd. Either way ull get a great car. Just different personalities. All I'll say is in two plus years I still get a huge smile each time I drive it! Good luck.