As you recognize, the decision has to be based on what you’re interested in doing. If you have a real passion for science, then go get a PhD and not a master’s. If you just want to be a patent attorney, forget the advanced degrees. If you want to be a patent attorney who does primarily bio/pharma work, then you need to think carefully about what you want to do.
If you really want to do bio/pharma patent work, then many say you have to have a PhD, but you should understand that right now, the national average time to get a life science PhD is 6 years (of very long hours and hard work). In the firms I’m familiar with, if you skipped the advanced degree and just put those 6 years into working hard and building your practice, you’d wind up doing more bio/pharma work than if you were new and had a PhD.
I have a PhD in molecular biology and biological chemistry. I currently work as a patent agent doing over-flow work for a large, general practice law firm. There are attorneys with and without PhD's at the firms I’m familiar with and I see no difference in their ability to get or perform biotech/pharma work.
Also, I don't think a Master's degree has a lot of value. My PhD got me the job, but I don't get to do much bio/pharma work because I’m not an attorney. Additionally, if in the future you decide to do something outside of law that is more science based, the master’s is not valued much since most master’s degrees are given to people who dropped out of a PhD program.
Although I enjoyed getting my PhD, if I had to do it all over again, I’d forget the PhD and go to as good a law school as I could.
Here’s what I don’t know about: getting an in-house job at a bio/pharma company without a PhD and working at a bio/pharma boutique law firm without a PhD.