My experience is they both give good service, so pick the one that feels best to you. If I had to guess based on my small sample size, the Ruger MK2 is probably a bit more reliable and durable than the Buck Mark. A buddy had a MK3 and I did not like it. He had to send it back to Ruger because it would not come apart and they ended up replacing the frame. I have a MK2 gov target model and can take it apart in my sleep, but I have owned it for about 25 years or so. My daughter has a new Buck Mark. Her first was lightly used and malfunctioned all the time. She took it back and has a new one that runs much better. Both are accurate.
I also find the Ruger very easy to take apart. It also does not require tools like the buckmark does, for a complete teardown..
