I totally agree with TC165: many saddles look good in a picture...then end up sucking wind. I had a really good opportunity to try on and actually climb in many different types of saddles when I was at Arbor Master training. Some saddles looked really great laying there on the ground, but after strapping it on and putting some weight in it: totally different story. Some saddles, no matter how many adjustments they have, just don't fit certain people. Anyway, here is my 2 pennies on the saddles you mentioned (cause I tried em both):
Master II
The Master II is part of the "ArborMaster Series" of saddles. I tried on a few of these, including the Master II. My complaint here is that the waist belt and bottom parts of the saddle are an integrated unit: that is, you cannot separate one from the other. Also, each one has its own limited number of attachment points (I'm talking lifeline attachments here, not tool/accesory). Further, the attachment points are not always located in the place you want them. Of all the saddles in the ArborMaster Series, I found the Master II the most appealing...but even it has limited attachment points (compare the Master II to the Pro Series w/ Pro Master Bottom and you'll see the difference).
.... then I tried on the Pro Series saddles.....
Pro Series
As alluded to above, one of the things I really liked about these saddles was the fact that that the "belt" is separate from the "bottom": they are not integrated. This appealed to me because I was uncertain as to which type of bottom I wanted to get: legstraps vs. bosun seat. There are two other characteristics of these saddles that are nice too:
a. lots of different lifeline attachment points
b. lots of different tool/accessory attachment points
I ended up getting the bosun seat and like it so far. I wanted a seat that was comfortable in a sitting position: could not get that from the legstraps.
I've played with various lifeline attachement points too: each distributes weight differently. I can't tell you how many times I've made use of the various attachement points when in precarious climbing situations: it is nice of have lots of choices based on your climbing position - for example, you can connect the lanyard to any of the lifeline attachments via a ******.
If you find yourself sitting on a branch while in the tree for some reason, the bosun seat can be a little unsteady: that is where the legstraps would be better. Some climbers I know say that footlock is easier with the legstraps too..... but I don't have any probs with footlocking and my bosun seat. If I ever really wanted to switch to the legstraps, it is simply a matter of swapping the bosun out: a 5 minute job and you're ready to go.
The only drawback to the Pro Series saddles is the price: get ready to drop at leat $300. Nevertheless, you get what you pay for.
Find a store that has the both saddles and check em out in person, bearing in mind the things I've mentioned and I think you will see how different the two saddles really are.