Raingear is an interesting conundrum. Trouble is, there isn't any raingear on the planet that will make you feel cooler than not wearing it at all. Many pieces are breathable, yes, but that's sort of a relative term ("more breathable than wearing a thick sheet of rubber!"). And they're not what you really need... That'd be water-cooled, with a little pump sending refrigerated water through tiny tubes built into the jacket.
So while I'd take a rainjacket, I certainly wouldn't take anything more exotic than your short-listed Marmot PreCip ($100; www.marmot.com) or REI Ultra Light ($125; www.rei.com). They're very similarso much so that I suspect the base material in both is identical.
And the fact is, in most cases you're going to be more comfortable just getting wet. So pack lots of fast-drying, lightweight clothing. Tropical wear, in other words. Columbia Sportswear's Silver Ridge ($40; www.columbia.com) is a good example: very lightweight, quick to dry, and offers excellent sun protection. Ex Officio's Buzz-Off Baja ($84; www.exofficio.com) offers all that, plus bug-killing protection built right into the shirt.
But don't buy too much before going to Guam. Just a few pieces. Then get there and look around. What are the locals wearing? Duplicate themthey know what works. And get an umbrella. It's a perfect rain shield in warm, wet climates, plus it keeps off the sun! GoLite makes a robust hiking-specific piece called the Dome ($20; www.golite.com), which won't take up too much room or weight in your duffel, true to that company's ultralight mantra.
For more expert reviews of wet-weather wear, check out Outside Online's all-new Jackets Buying Guide.