Radar is a great tool if you find yourself running in fog or night time often. Like any tool, you need to know how to use it to get the most out of it.
With your boat, you don't need anything massive. Idealy a 4kw dome would be just fine, as you are probably only concerned with objects in the way, nav markers, and other boats.
If you are on a budget (like 99% of America), I would suggest a furuno 1715. It's a nice little stand alone radar, b/w LCD screen (no point in having color), and extremely easy to use. Dispite the fact that its only a 2.2 kw set, the results have been damn good to my eyes. Leaving the Maurice River, we can see the DE coast and the NJ Cape perfectly. Seeing boats 8 miles away is no problem.
I have many friends that have this unit, and they all are tickled pink. The best part is it won't break the bank, usualy around 1700.
As far as having two antennas... that's a toss up.. some will argue yes if you can switch them on the radio, some will have two radios and one antenna. I have always felt a good radio and a good antenna can't be beat for reliability. A handheld in a ditch bag will work a lot better if you find yourself offshore with a dead battery!!