I am looking to get a new sonar unit, hopefully under $500.00. What is perhaps the most important aspect of a sonar unit. Right now I have a bottom of the line humminbird unit. What's better and why??

For me, it's 3d

I agree with LBH.....I learned a whole lot from a Humminbird 3D wide unit I once owned.It gives you a very detailed 3D look of the bottom that you are fishing.It will help you find the sweet spot faster.

For $500, you are very close to getting one with a GPS  combo built in.  If you fish big waters, the addition of a chip that shows contour lines are invaluable for finding creeks, humps, and other fish holding features quickly.  You'll have to purchase a chip for your region, but many like the Lowrance with the 480 x 480 pixel count unit.  I think it can be had for around $600.  If yo fish small waters, forget the gps idea.

Are all the other companies offering 3d? I can only seem to find the Humminbird model. Thanks!

I have 2 lowrance units on my boat,neither are 3d. I have fished from a friends boat that has the 3d finders and I actually like mine better,maybe its what i'm used to but I read it better. Make sure you buy at least one color unit. I like the color units by lowrance. For this time of year it is very easy to pinpoint thermoclines with color. As far as GPS goes if you dont fish any really big water its not needed. I have one on my boat and it is an $800 speedometer ;D , thats about all I use it for.

I think Lowance has one model that is 3d.

that would be the iWAY 500C , but it's an in-car navigation unit, not a fishfinder unfortunately.