Short Answer....Cozumel is nothing like mainland Mexico. I have never been to Hawaii, so I really cant compare the 2. I can tell you lots about Cozumel though.
We go to Cozumel at least twice a year and hope to do at least 6 months a year within 10 years.. I will guarantee you it will be cheaper. We do not stay at the all inclusives because there is so much good food in town for cheap to very reasonable prices. We stay at Casa Mexicana for 70 to 80 a night which includes arguably the best breakfast buffet in town. Very clean hotel with nice rooms and excellent service. Comfortable beds too.
Dive-Op wise, Dive with Martin will probably be the most cost effective shop on the island around $60 for a 2 tank dive if multiple dives are booked. Aldora will be on the higher end around $95 fro a two tank dive, 10% discount for cash, and they use steel HP120 tanks for 80-90 minute dives. There are other benefits to justify the expense for Aldora as well, just check out their website. The remainder of the ops will likely fall into the $80 range for a 2 tank morning dive with aluminum 80 tanks..
Stay away from cattle boats with more than 8 divers on board. There is no reason to dive with 20 other people when the smaller, faster, "valet" service type shops are out there. We dive with Aldora, but there are dozens of top notch operations on the island. We have dove with Dive with Martin and Blue Magic and can recommend them both. Blue XTsea, Tres Pelicanos, also have a huge following on the Cozumel site as well.
Diving wise, most days there is incredible visibility, an abundance of aquatic life from eels to huge turtles to sharks and everything in between. There are usually mild currents so your are truly drift diving, very little finning is necessary. We just got back from Roatan and the reef was healthier there, but very limited aquatic life and no where near the visibility of Cozumel. If you want to go fast, go to the North end...zoooom! Safety wise, our first dives were in Cozumel with my 12 year old and 18 year old daughters. I was never concerned about their safety.
Town wise- we love it. We have walked all over Cozumel after dark and never felt threatened or afraid. I'm a little paranoid by nature and never been in a situation where I got a bad feeling. Generally, the people are very friendly and willing to help if needed. I can get by in Spanish, but have never needed it. My wife does just fine and her only Spanish is the menu at Taco Bell. Parts of town may look less than appealing but we have never had an issue. Cabs are cheap anyway, usually $4 for most in town rides, cheaper if you have Peso's.
If you want an all inclusive 5 star beach experience, you will need to go to the South end of the island. If you want the Cozumel experience, Town is the place to be.
Not sure what air fare would be from Oregon, but one of the reasons we love it is we can get there from OKC for 200-300 regularly. I would highly recommend going in October/November for Day of the Dead. Its an amazing time and absolutely a cultural experience when the island comes out in the square for the festivities.
Good Luck, safe travels
Jay