Honestly, I think you're overly concerned on this one. You're not going to choose one and find it to be utterly inferior to the other. Each brand has its defective models that unfortunate buyers out there have told horror stories about. Each one has had corrosion issues as well. Both seem to have remedied the corrosion problems, whether by re-design or by replacement of the faulty units.
Really, if you have access to a Merc dealer and a FW dealer that are reasonable distances, then just get the best deal. I think I'd do some homework to find out what the Merc dealer and the FW are like to deal with. Do they stand behind their product, quality of work, are they easy to deal with when it comes to down time, do they try to charge you for/perform work that is unnecessary?
I think these questions are the best ones to have answered in making your decision. If one of them has a poor reputation, it might make your decision for you. If they both are equal, then shop for what you feel is the best deal overall for your money.
And don't apologize for asking questions. Information is the enemy to someone who is trying to get you to pay too much or make a poor decision. Get as much of it as you can!

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Paul
2007 Four Winns Horizon 220Mercruiser 350 Mag MPI w/ Bravo III Drive
2004 Ford Explorer Limited (V8-AWD)
Pomme de Terre Lake (Southwest Missouri)