Stains (even oil based) are not really that water proof & wear quickely when exposed to water & the elements (my outdoor stuff is only good for 2-3 months).
I used 3 coats of poly for my stand. Flat poly is pretty dull, but still looks a bit platicky. If you are into the natural look, I suggest sealing all the inside & hidden parts with poly. This area is what you want to protect anyway - water outside will dry pretty quick, but water in hidden area may soak in & losen your joints or rot the wood.
For the outside, try Danish oil (I use Watco Danish finish). It keeps the natural feel of the wood & is fairly water proof. Only problem is that it will wear over time & you need to redo it every few years. I've also use water based poly (Behr), not nearly as plasticky as oil based poly, esp. after lightly sanding with fine sandpaper. But you'll need to use water based stain with it.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.