[neilc]

Most PhD and MS programs are very different -- I wouldn't try to substitute one for the other. Many good schools (e.g. Berkeley, MIT) don't really even offer MS degrees, and many MS degrees are coursework-based (e.g. Stanford's), for which you have to pay tuition. You're basically there to continue your undergrad: take some more advanced classes and perhaps do a bit of research, but mostly it is like extending your undergrad degree. Whereas a PhD is entirely about learning how to do research. Which degree (if either) is the right one for you depends on what you're looking for, but they are definitely not substitutes.