I use sanke's but its only because I got a great deal on 5 1/6 sanke kegs and the coupler's to go with it. People say the benefit to corny's is that they are easier to fill and clean. Well, it takes me about 30 seconds to disassemble and 2 minutes to assemble a sanke keg. As far as cleaning, I do it the same way every one else cleans their carboys, fill with Oxy Clean, soak, rinse.
If I was doing it again, I'd probaby use corny's but only because they are easier to find. I had to scour craigslist to get up to the eight that I have. One benefit is that it universally fits the standard kegerator if you ever want to take your beer on the road.
Regarding your questions, here's my opinions:
1. I think it would be the same for either.
2. About the same, the two connections for corny's are ~$12 each and a sanke coupler is about $25.
3. Yes, you can prime and you can do it in either.
4. Don't know. I will say that it is super easy to tap and untap sanke's but the installing the snap ring can be frustrating the first few times.
While I expect this thread will overwhelmingly lean towards corny's and I understand that, I also know that there are alot untruth's going around about the difficulty of sanke's. I personally think that other than opening and closing them, everything else is easier in a sanke.