First, chances are you will have to wait at least 6 months to put any finish on pressure treated wood unless the treatment was a type that can be finished immediately. Check with the supplier. You will just be wasting your money and potentially causing problems if you race the process.
Since it has not been finished before, when it is time, I would stain it. Your options range from semi-transparent stains to solid stains. Semi-transparents preserve some of the existing color and most all of the wood grain variations. On the other end of the spectrum, solid stains will leave the wood grain texture but will be opaque like paint. People are probably getting tired of me posting this but it provides a nice visual explanation.
The argument is getting harder but I preferred solvent based semi-transparent or even semi-solid stains. If you live somewhere where solvent based products are hard to buy, you will probably do alright with waterbased stains. I think when you get to solid stains you are better off with a good acrylic product like Sherwin Williams Woodscapes. Woodscapes can be tinted to any color like paint and the acrylic provides excellent colorfastness. Ben Moore makes a comparable product but it is fairly new and I never used it even though I used their paint more than others. Make sure you use a product with nice UV protection built into the formula!
You could also use a quality primer and paint but with solid stains what they are and your ability to match any color, it doesn't make alot of sense in my opinion.
As for how often? There are lots of variables starting with just the kind of weather it is going to be subjected to on a regular basis. If you use a good paint store product over properly prepped surfaces you might get 3-5 years out of an application. I usually advised folks to plan on 2-3. If they got more great, if not they were prepared.
Do avoid wax based sealing products like Thompson's Water Seal. Use will come to haunt you.
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