which is a better car - mercedes e350 convertible or bmw428ixconvertible?

These are both good cars. If your budget can afford it, I'd go with the Mercedes.

If there both new vehicles it won't matter for 4 years because or factory warranty..... If not the BMW has a lower cost of ownership

Cars mean different things to different people. Some will like one and some will like the other. Both are nice, but always remember, German and English vehicles usually require more maintenance and expensive repairs than the Japanese cars such as Acura, Lexus and Infiniti. Never own a Jaguar, Mercedes, BMW, Audi or Land Rover with out a warranty.