Hard to say...a newer MB versus your older Lex?
Did you buy the Lex new? If so, you tend to hold onto cars a fairly long time. MB drivers tend to cycle through them via leasing. I presume the one you're looking at has come back off a lease. I'd then presume you'd keep this for a while too. No matter what, cars cost more as they age...in maintenance as well as residual value.
Your Lex has probably given you many years of trouble-free miles. Are you ready to dive into the sports car realm (of MB no less) and the potential for higher maintenance costs? I agree there's very little that is sporty about a Lexus and having a newer more fun car is refreshing, but I wouuld plan on more money devoted to gas and maintenance than your current Lex.
The short answer is, "love will find a way". But hopefully I gave you a few things to be aware of when plunging into the world of MB.
Good luck and let us know how it goes!