It’s a very open ended question but a good one. The titles are what you make of them. A Master could be anything and so could a Sir. But I’m assuming this isn’t the answer you’re looking for. Let me preface what I’m about to say with the fact that this is just based on my personal experiences and opinions and that it in no way sets up a frame of references for any Sir or Master. Speak to the Dom to understand what it is they are all about!
In my travels, most people who refer to themselves as Sir are people who put a high price on domestic abilities and more of the gentler kink. There seems to be more flexibility and less of a desire to dominate completely. They tend to let their subs act and speak more freely. Now…Like I said, this is not true across the board. I have met Sir’s who simple like the sound of ‘Sir’ better than ‘Master’ when they crack their whip across their slave’s ass. Master does seem to hold to a certain level of power though. When you have a Master you tend to have a slave or pet on the other end who have a little less say in the daily going on’s.
That is my persnal opinion, though as I stated more than once, there are exceptions to these rules and I’ve found that most Dom’s pick what they want to be called simply because it appeals to them on some level. I’ve met people who liked to be called ‘Marquis’, ‘Lord’, ‘Prince’ and many other titles.