The extra power, comined with the slightly higher resolution will make for a much more sensitive pick-up. There was a post on this a while back, and some seemed to agree that the exta wattage, made it more possible to pick up things like your line and lure more so than higher resolution. The color makes a big difference (I wish mine were color). And the GPS of the 28 is great too. I would say it is well worth the extra money. For that kind of money, I would look into one of the hummingbird side scans - those are cool!
The LMS 520 is a cheaper option if you were looking for something with GPS, color, and cheaper.