You will want one of the models with a "w" in it. In addition to having WAAS capability, I believe these are the newer units which have the better, sunlight readable displays.
I would suggest you find a retailer in your area that has one that you can touch, see, play with menus, etc.
I compared the 1850WDF and the Garmin 2006C with GSD20 sounder this spring. From what I remember, the 1650 to 188 comparison will be very similar. I chose the Garmin for these reasons:
1. Garmin was easier to use for me ( I had been using an old Garmin 210, so some of their logic was already familiar to me).
2. The Garmin display was still brighter, clearer and easier to read in the sun IMO.
3. The pop up menu's on the Furuno's take up a fair amount of screen on the right side whereas the Garmin pops up as a narrow band across the bottom, leaving more of the screen viewable.
4.The Garmin was a little less $$, but it was fairly close at the time (special boat show pricing) and I really only looked at the price after other considerations (I would pay a little more to get what I want if it's something I'm using every trip for the next 5-8 years).
5. Always had good customer service from Garmin.
6. I like the Mapsource CD mapping program for use on the Garmin products. Very useful in waypoint management.
Both are quality companies and I think it largely comes down to personal preference.
To answer your question on the 1650/1850 line:
no letter = standard GPS
D = differential GPS
F = fish finder
W = WAAS GPS
Some are combinations, like WDF has WAAS, differential and fish finder.
Furuno 1650 Model Line 
