Just personal preference in my mind. I prefer the one piece. 3/5 should be good, but then again I have been classed a polar bear because I'll wear a 3mm in 52 deg water (hood and gloves a must though). Even the people at work think I'm nuts
I see one piece and two piece wetsuits. I recall using the two piece and liked it just fine. Is there an advantage of one over the other or is it just a matter of preference?
I'd go full mask, just one less thing to worry about in case you find a stream with a nasal variant of the candiru.
I also noticed a couple of masks that don't extend over your nose. This type of mask interests me since my mustache creates a leak path for water to enter my mask. I wonder if anyone here has experience with this type of mask and if it might be suitable for riffle snorkeling. Any pitfalls to this type of mask (aside from water in my nose)?
I have the full beard and mustache and have no leakage problems with my mask. Best bet is go to a dive shop and try various masks. Put the mask against your face (don't put the strap around your skull), inhale through your nose and look down at your toes. You should be able to find one that holds the seal and then you have a winner.
And never forget, temperature is a state of mind untill your brain goes numb and you forget who you are.