"and I'm looking for a filter to protect the front element"
Don't. Add a lenshood and keep the front cap on when you aren't shooting.
Any filter will compromise the optics of your expensive lens.
"I've decided to go with B+W since they seem to have the best quality to match my L lens."
That's the right thing to do for any filter, especially if it's going to be mounted on some expensive glass.
A lot grade filter will turn that expensive lens into something that's no better than some cheap knockoff.
Put it like this:
A cheap UV filter is just a piece of thin windowpane. You're thus effectively shooting through a window.
An MRC version has a lot of the glare reduced that that produces, at the cost of some colour cast. It's also likely hardened so it doesn't scratch as easily (and those filters will scratch a lot easier than does your lens).
If I use filters at all (and currently I use ONLY polarisers and the very occasional Skylight 1B) it's always the best grade I can get in the size I need. And if that's not the best on the market it gets marked for replacement by something better the moment I get it (thus I'll only buy cheaper versions if I need it urgently and can't get better).