I have a vinyl 18x36 pool and would ditch the sand filter in favor of a DE filter. Bit more work but worth the effort in my opinion. I am swapping out my sand filter in the coming weeks. I would also consider the Pool Pilot Digital SWG. They seemed like the best to me for features and value and poolsean is a regular contributor to the forum who works for the company.

As for the Dolphin, well I have had a Diagnostic for 3 years. I gave that one to my Dad and bought a Dolphin Dynamic from Costco. It works great in my pool. It's not perfect but what cleaning system is? It gets 90-95% of the dirt and garbage and I don't have to deal with any hoses. I bought a Poolbuster for the light clean up jobs and for my spillover spa. There are other contenders for robotic pool cleaners (ie Aquabot, Blue Diamond, Blue Pearl etc) but in the end I believe they are all the same. Some people have had problems with Dolphin but it's not unlike any other purchase. Some will have issues while others won't. For example, I spent $75K on a brand new car that burned a quart of oil every 500 miles. I'll never buy another because the manufacturer refused to fix it but there were thousands of other people with the same car that had no issues. Bad luck I guess. Either way if you do buy a Dolphin buy it from Costco as they have an astounding return policy.