Hm, looking back at my pumidor and my decision-making process... I wish I had gotten a bigger one!
So I would recommend starting with the size as your primary planning factor. That might seem like a 'duh' statement, but the hardest part will be trying to estimate your 'fill rate' so that you can have enough room to grow.
I got my pumidor relatively early in my pu-life, and I greatly misjudged my need for space...!
Other than that, depending on where you want to put it, form/style is probably the next consideration. If you can stash it somewhere out of sight, then it really doesn't matter what it looks like...
Also keep in mind the humidifier mechanism. If you go mechanical (like the one in the link), they do make noise. They're not always on, and the material of the pumidor may (or may not) help muffle it a little... but it depends on how much whirring sounds like that bother you.
But size. Definitely start w/ size.
