The handhelds are nice because you can take them out and use them for hunting and in your truck. That's the primary benefit to me. The drawback is that the display is smaller and therefore harder to read when running in rough water.
I would recommend the Lowrance Ifinder H2O. It's a great unit, VERY accurate, tons of options, and you can put the MMC chips in it with programmed maps on it. I used Buzz's last year when mine crapped out and it was awesome. I'm getting one soon.
If you do go with a fixed one, stay with Lowrance or Garmin. Both are good.
On the handhelds, mount them using a RAM mount in the boat, but be sure to put a tad of electrical tape around it in the mount to keep it from popping it out in rough water. Anything up to 3 footers and you're fine. But in really rough water, if you MUST run (say in a tourney) then make sure she stays put with a little elec. tape. It doesn't leave any gunk or marks on the unit either.