You guys forgot to mention another important factor: resolution. The more pixels, the better the representation of what's down there. Say you're in 40 feet of water. A screen that has 160 vertical pixels is going to show 1 pixel on the screen for every 4 inches of depth. That means if you see a fish at 30 feet down, it's going to show up as at most 3 dots on the screen. If you had a finder that had 240 veritcal pixels, you'd have 1 pixel for every 2 inches of depth, giving you a much better picture of what's down there. Now, say you had a finder with 320 vertical pixels. You'd have 1 pixel for every 2/3rds of an inch of depth. This goes for structure too. The more pixels you have, the better the representation of what the sonar itself is actually 'seeing'.

If you're not hung up on getting a Garmin, I'd suggest looking at the Eagle FishMark 320 or SeaFinder 320DF. I just got a FishMark 320 myself from Cabela's, and it arrived in the mail yesterday. While I haven't been out on the water with it yet, I've played around with it extensively, both my real one, and the emulator Eagle has up on their web site. I must say, it's quite impressive. 320 pixels, and 1500 watts peak to peak. The SeaFinder 320DF has the same specs, but higher wattage and dual frequency, 4000 peak to peak. I got mine for lake fishing, so I went with the FishMark as it was a little cheaper, and was the only one offered in the portable version, but if you fish saltwater and have a regular boat, then you might want to look at the SeaFinder. The FishMark 320 runs $200, and the SeaFinder $240.

Here's the link from Cabela's:

FishMark 320 and SeaFinder 320DF at Cabela\'s

And here's the link to emulator software for both Eagle units:

Eagle sonar emulators