I am looking to spend around $250 on the unit. I need one that is water proof , has a long battery life, and easy to use.
You also forgot to mention having the largest color screen, lightest weight, and most features.
I would add to what ISP2605 said a couple of additional points. Some of the handheld units do have highway maps, but aren't great for having the unit calculate how you get to a destination because of the tedium of inputting address information. So while it may be good for telling you where you are, most of the little guys are not good for telling you how to get places because of the data entry tedium.
LCD displays turn all black when they get hot and so some won't be much use if left in the sunshine for too long. Once they cool, they work fine again.
Another problem wtih some is that they can be complex to setup. For example, some are designed such that you download the map(s) for the region where you will be. That means learning the software and downloading procedures. The benefit here is that you get the most up to date information available, but things like highways, mountains, etc. don't change much over a few years which will likely be the use life of the unit. So a preloaded maps unit may be a better way to go.
Garmin and Magellan are the two main brands. From what I can tell, Garmins seem to be a bit more user-friendly, but Magellans tend to be technically better GPS units. The question is whether you are happy with being 50' accurate or 10' accurate.
If possible, I would go with the preloaded maps and a color unit, but I don't know if that is in the price range. I have had both the LCD and color units and you can just get more visual information from the color units than from LCD. It will have a small display, however, as is the nature of the handheld units.