This is one of those times where a "one size fits all" answer isn't the answer. What works best for you may not work well for someone else. That said, here are some observations:
First, if you're looking to save energy and help the environment, please remember that a lot of energy and resources were invested in the making of your current car, and at 150K miles its next stop will be the scrap yard, not a new owner. A lot of that invested energy will go to waste when you trade your current vehicle in, so to be kind to the planet it makes a lot of sense to keep driving the older vehicle until it's time has clearly come.
Waiting to trade it in will have other benefits, too. Two or three years from now a new crop of hybrid, diesel and gas vehicles will be up for sale, so there will be more choices and more competition for your dollar in the fuel-efficient SUV market segment.
As for which technology -- hybrid, diesel, gasoline -- makes sense, that depends a lot on how you plan to use your vehicle.
With all of it's stop-and-go driving and idling at stoplights, Hybrids are a really good choice if you do a lot of city driving. If this is how you do the majority of your driving and want an SUV, the current Mariner/Escape Hybrid (same Ford platform, different trims) is a pretty good choice, just keep in mind that it is not designed to tow a trailer, not even a small one. (So, if your idea of economical retirement and vacation fun is
towing a small camper-trailer, you might want to wait a few years before you buy to see if a hybrid or other fuel-efficient technology SUV that can tow becomes available.)
If the majority of your driving is done on open freeways (not in traffic), I'd consider waiting for a small diesel SUV. Diesels are hard to compete with when it comes to efficient long-haul driving or towing a trailer. Current diesel VW Beetles crank out 50+ mpg highway, and some have even been outfitted to tow (very) lightweight trailers. It also looks like there will be more efficiently produced bio-diesel fuel options a few years out, and that's worth considering, too.
The downside of diesel compared to hybrids is their engines have to be left idling at stoplights, and that's energy and emissions up in smoke without any productive return. Diesels also can't store energy recovered through regenerative braking like hybrids do -- more energy and emissions without productive return.
The last choice is a "flex fuel" gas engine that can burn ethanol blends as well as traditional gasoline. Right now it's not my favorite choice because there are no efficient ethanol bio-fuel production options. The ethanol made in the US from corn requires almost as much energy to harvest and produce as it provides to the consumer. (Brazil's sugar-cane ethanol, on the other hand, has a better than 4-to-1 energy output per unit of energy put in.) New production crops and technologies may allow us to produce ethanol from cellulose fiber (instead of corn sugar and starch), which would allow us to switch from corn to other crops (like sawgrass) that require less energy to grow and harvest.
The problem with cellulistic ethanol production is a finding a method for translating experimental "lab" production methods into industrial scale production has been "just around the corner" for a long time now. The difference right now is there are a lot more people and dollar investments working to make it happen. So, if it is possible, I'd expect someone to come up with a method for it sometime in the next few years, and when/if that happens, flex-fuel gasoline engines and hybrids may wind up being the wave of the future.