I've been freediving for about 2 years and have just got my scuba ticket this month. I'm not the best freediver, and most of my photography on snorkel was done under 5m.
The differences between the two was brought up on flickr a couple of weeks ago, and this was my response:
Snorkel Pros:
1.\Unlimited air, so unlimited time to stay in the water and find critters and photograph them.
2.\ Access to shallow waters such as rockpools etc that you'd just look like a fool accessing on scuba.
3.\ More ambient light in shallower waters increases lighting options.
Snorkel Cons:
1.\ Diving down on fish scares the hell out of them.
2.\ Reduced time to compose, alter camera settings and strobe positioning etc.
3.\More difficult to get eye level and surface shots.
4.\ Severe spinal contortions. 
Scuba Pros:
1.\ Approachability to marine life
2.\ The ability to stay still and let critters come to you.
3.\ To get to depths to see different subjects not possible on snorkel e.g. wrecks etc
4.\ Unlimited angles.
5.\ Time to review your photos and change settings, lighting etc.
Scuba Cons:
1.\ Bubbles - You really shouldn't breathe hold either which aids in focus lock and composition. Also bubbles get in the way of the viewfinder and vision
2.\ Time and air restrictions.
3.\ You need to get tank fills, more gear setup time, planning, risk etc on scuba.
One thing i didn't add to the above which i'm starting to realise the more dives i do, is the reduced field of view and awareness of surroundings. My scuba dives so far have generally been horizontal along the seabed, and i've been finding my awareness of my surroundings is limited to what's in front of me, and in my periphery. On snorkel, i find i'm pretty aware of everything beneath me, plus whats going on above the surface (to a degree).
I think each has its place, and i plan to utilise both skills into the future (once my scuba bug has subsided. 