There's no one sander that's right for all jobs.
If you're doing detail work, you might spring for the Fein MSX-636-2START Multimaster Start detail sander. This unit has a sorta-triangular sanding pad that gets into corners well. A great accessory is the 3-1/8" Sawing Blade that cuts hard things, like fiberglass, but not soft things, like hands. That's because the blade doesn't rotate, it vibrates. The Flush Cut Wood Blade is similar, but offset for getting right next to an obstruction. As with most tools, buying a kit can be a cost-effective starting point.
For sanding curved surfaces, you might prefer the Porter-Cable 9444P Single Speed Profile Sander or the 9444VS Variable Speed version. These come with a bunch of rubber shapes to hold the sandpaper in a variety of interior or exterior curves. There's also a flat sanding attachment, but it's not as maneuverable as the Fein sander. The Dremel 6000 Contour Sander Kit is a less-expensive tool with the same concept, but I don't have personal experience with that one.
For basic light-duty sanding, consider the Black & Decker MS500 Mouse Sander/Polisher, also available in a kit with accessories. The Black & Decker MS700K Mega Mouse 4-In-1 Sander/Polisher is a newer version. No, I don't know what the difference is.
For bigger work, such as fairing new fiberglass repairs, I recommend skipping the 5-inch units and going to the more powerful DeWalt DW443 6" Heavy Duty Electronic Variable Speed Right Angle Random Orbit Sander. (Actually, I have the model DW444, which is the same except it takes pressure sensitive adhesive (PSA) sanding disks instead of hook-and-loop.) These units can save you a lot of time getting that new patch of deck smooth. They're also suitable for woodwork, as the power can be turned down. And the electronic control means the power level doesn't change when you put the unit down on the work.
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