Let me preface this by stating that I've been terribly spoiled by my Ocean Master flex dry snorkel. Aside from the bulk and the need to have it dangling on the side of my head if I plan to use it, it's a great snorkel. It stays upright when I'm face-down and it's totally dry.
I bought the Aqualung Snap because I had taken to diving without a snorkel as I didn't like the bulk and the weight. As far as I'm concerned:
The Snap rolls up into a holder that's about the size of a hockey puck. Very nice and compact.
The Snap also is relatively easy to remove from the holder and attatch to your mask.
However, I found that the Snap didn't always want to stay totally upright and, thus, wasn't that dry. It also lacks any type of purge valve, making it more difficult to clear once water gets into it.
The result: I'll use my Ocean Master when snorkeling or when I want a snorkel due to rough surface conditions.
I'll take my Snap when the surface is calm or when the only reason I want a snorkel is for legal reasons (some places require you to have a snorkel). If there's a REALLY pressing need to use it, I'll have it but I won't be using it by choice.