I've used the Vexilar for years. I know it best, and they work fine. I'm pretty sure Marcums are also good. your choice. Base your purchase on your intended use, and the associated features. Been using flashers since the Lowrance "Green Box." Use your flasher only as a tool, and don't get too hung up on it for most of your info. Nothing like a lot of time on the water.
I think the marcum has more "power". I have also heard the marcum and vexlar interfere with each other??? Not sure if thats true. If it is buy whatever the people you fish with the most have. Both of them are good. I hear hummingbird is stepping up to???
In terms of putting fish on the ice I don't think there is much difference.
Like said before I believe the Marcums have more power but I can't say the extra power really makes them better. The Marcums are also know for interference issues. They can filter out interference fairly well but they create a lot of it for other people around you. So if you buy a marcum and fish with a bunch of guys with vexilars you'll probably render their units useless. Not a good way to make friends. I know my group usually runs vexilars and have had to tell a new guy to turn his marcum off because he was interfering with all the vexilars.
I think its good advice to simply buy what your friends use to avoid the issue. If you only fish by yourself then buy whatever unit you can get a good deal on. They are all good units and will help you catch fish.
If you've got short, stubby fingers and wear reading glasses, any relaxation you would normally derive from fly fishing is completely eliminated when you try to tie on a fly. ~ Jack Ohman, Fear of Fly Fishing, 1988