I've been looking into quite a few programs with MS and have even considered a straight shot to PhD. I actually spoke to a program director at a school yesterday where I'm considering pursuing a master's degree in marine bio. Here's a summary of the discussion that I had about the differences between PhD and MS: If you want to do more hands-on stuff like lab work and fieldwork, MS is the way to go. PhD seems to have more about creating research projects, writing proposals and some administrative stuff. In addition, at the universities, they teach classes etc. There's some salary differences naturally but some of the working situations can be comparable.
I think a con to pursuing a PhD without a MS could be that, similar with any professional degree, if you change your mind while you're knee deep in it, you haven't got much. But, if you're positive you want a PhD, go straight for it, I suppose.
I also know that some PhD programs have you able to gain a MS while pursuing the degree while other PhD programs REQUIRE you have a MS before starting. It kind of varies. I was talking with a friend yesterday who told me that "some employers may look down upon getting only a Master's degree in a discipline in which they also offer a PhD, because they think you couldn't hack the PhD program. So if you only want a Master's, it'd be better to go to a school that grants a Master's as its highest degree, rather than a school that has both." It seemed like a valid point to me.
For me, I'm considering applying to a MS program that really interests me and has a lot of the criteria that I want but I also am considering applying to a PhD program at an awesome lab. I'm at the point where I want more exposure and experience to be a good scientist. Although I worked and strove (?) to get into vet school, I ended up wondering if it was what I really wanted. I'm still muddling through that but am getting closer to figuring it all out. At the very least, I'm grateful that I'm a trad. student who has a lot of time ahead of me to fill up with being a student.

Hopefully I can figure it out
Best of luck to the OP with deciding! No matter what you decide, I am sure you will do well if you put your effort into it.
Side note: I explained my situation to the director and he shared a story with me about a girl who had a 4.0, was from a big research undergrad, but was set on vet school. She more or less said such in her PS and she wasn't accepted into the program cause they saw where her true interest was. And it was not in their master's program. So, I will keep that in mind when sculpting my PS part deux.