I'm going to repeat what others have said, for the most part. They are both quality units that will help you catch more fish, they are both backed by reputable companies with good customer service records, and both brands hold their value.
Personally, I have used both over the years, along with the Hummingbirds, Lawrence and Zercom units, I prefer the MarCums. I think that they simply offer a better value for my hard earned dollar. MarCum's lower priced models have the same or better features than Vexilars higher priced models. Value aside, I think the MarCums do a better job of "shooting through the ice" and I prefer the display on the Marcums. Bean has a very good post on both units in the General section, which offers a side by side comparison of both models.
