Howdy Lance,
I don't have any experience with the two models you listed, but I installed a LMS-337C-DF on my rig three years ago, and I love it. The only problem I have had with it was when I purchased a Navionics chip and tried to install it. I had to contact Lowrance to get an upgrade chip for my unit. It seems that in the course of one year, my system was outdated. Other than that wrinkle, I have nothing but good to say about Lowrance units.
Bigwhiskers