I am familiar with Guam and Hawaii (Oahu). I lived on Guam from March 92 to December 94. I am currently on Okinawa and visit Hawaii for business a couple times a year. I am looking for orders to Hawaii the first of the year and plan on retiring there myself.
Both can be on the expensive side, but, if you have that ID card, you won't notice it quite as much. Guam is a bit more isolated. I would consider retiring there myself if I got re-married, but I wouldn't do it single. They do have a K-mart now, but not much else in the way of shopping as we are used to in the states. Also, the exchanges and commisaries (2 each, as compared to 3 large ones on Oahu) tend to be out of stock a lot. I think you will find the medical care a little better in Hawaii as well. The Navy Hospital on Guam gives great care, but they are small, and service is limited. I don't recommend the civilian hospital there. And while Guam is a territory, it is not a state, so their medical care does not fall under JACHO and FDA.
Guam also gets more (and more severe) typhoons than Hawaii does hurricanes! And for some reason they can't seem to get their infrastructure up to par. They just got hit with a pretty nasty typhoon and they are predicting to have full power restored...in 8 weeks! Also, at least when I was there 10 years ago, they could not power the entire island 24 hours a day, so they did rotating "load shedding" where certain parts of the island would be without power at certain times of the day. But honestly, that was part of the fun of living on a tropical island!
On the other hand, Guam has by far the better diving as compared to Hawaii. And the water temps are much warmer (82-84 year 'round). And the diving and dive gear are MUCH cheaper than I've found on Hawaii.
Also, when I was there, there was no income or sales tax, although, I think that may have changed. I was there on business last Dec, and the housing prices had really dropped since I had lived there, so it wouldn't be a bad time to buy a place there. Beware renting though. I arrived at the time the Subic role back was happening. Housing prices had doubled in a couple months time. I think there are going to be some subs homeporting there in the near future, which means prices will be on the rise again.
As I am expecting orders to Hawaii for the first of the year and having been checking out housing there. To buy, it is actually cheaper than San Diego, and about the same as DC right now, plus, they have about the lowest property taxes of any state (not sure how that compares to Guam).
I guess between Guam and Hawaii, you would be doing more "roughing it" on Guam!
Good luck, and enjoy retirement.