Personally ... if I was going to spend $500 on a depth finder, I'd opt for DI & the biggest screen that money would get me. But, it might also depend on what styles/methods you use. It may also depend on where you intend to place the transducer.

You might also want to talk to Brian@BBGmarine (Member Sponsor Classifieds) and see what he suggests for your style of fishing & budget. I haven't dealt with him, myself, but I've heard a LOT of great reviews on his pricing, customer service, and shipping time.