I replaced my radiator with a new Behr back in May and was disappointed to see no reinforced neck. The original Behr that I replaced was also not reinforced.
A few months ago there was a post here indicating that Behr was no longer selling radiators with reinforced necks? I do not know this to be true, but I purchased mine from an MB dealer for just over $200 and again, it was a Behr and it came without the metal sleeve I've heard so much about.
Over on the Ritter-Easley list I've read mixed feelings about Nissen. Some have reported higher operating temps; others say they're fine.
Personally I'll stick with what MB put in the car - Behr.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.