Stan,
I think it all depends on where you mount the hand-held, how sharp your eyes are, and how good your concentration is. I have a Lowrance I-Finder Hunt + hand-held that I put in my truck, and if I'm on a bumpy road, it's hard to read it - it's mounted in a Ram mount on my dash. The same went for my old boat - I had a hand-held Lowrance unit - can't remember the model right now... but if I wasn't in absolutely calm water, I couldn't read the unit and drive safely at the same time. With my 332C mounted in the dash of my boat, it's right there in front of me - not slightly off to the side of my console like the hand held was. It also is a color unit that has a very sharp picture, and my eyes focus on it, and read it better, no matter what speed/water condition I am in. I much prefer the 332C mounted in dash over the hand held mounted to the side of my console. I also have aging eyes, so the larger display of the dash mount is better for me than the tiny screen of a hand-held.
On the issue of having seperate units - I've had seperate units before. They have pluses and minuses.
The plus side - If one unit breaks, you don't lose both capabilities - GPS and Sonar.
The minus side -
1) more room required,
2) more attention to focus back and forth between where you are driving, then sonar, then GPS, then back to where you are driving..... The one unit in the dash that is set up correctly - shows me where I am going, the depth in a DIGITAL readout (nobody is reading depth contours at 70mph anyway), speed, temp, etc.... at a glance to the dashboard. When I slow down, I am reading depth contours and bottom structure, as well as pinpointing my position - comes in real handy when checking out grassbeds.
3) costs - it's cheaper to buy one combo unit than two seperate units
4) battery drain - it's not a big factor, but running two units drains more power than one.
So my vote is a single combo unit in the dash. I still have my bow mounted sonar too.... so I'm never without. 332C gets my vote for the dash, with a 102C up front.
All the best,
Glenn