I got a PhD in biology (genetics) straight out of undergrad. I decided to go to law school when I realized that what I wanted out of my career--a stable, fairly high-paying job in the city where my family lives--wasn't going to happen in science. I can't recommend that anyone get into a field that requires 5-8 years of grad school and 3-10 years of postdoctoral training, all in preparation for a slim chance at getting an ok-paying faculty or industry scientist job in a geographic region not of their choosing.
A J.D. is more practical because law school is about getting a job in 3 years. You make resumes. You have summer jobs. Career services meets with you. The people who do well at decent schools have their pick of six-figure jobs in the cities of their choice, three years after they start (that's what happened to me). The people who do just ok might not have as much choice or money, but they get jobs too.
In a bio PhD program, no one really talks about jobs. Most people enter their PhD programs with little or no understanding of their long-term goals. They drift through their Ph.D.s in denial about the fact that they will ever have to get a job. They have a vague sense that after their PhD and a postdoc or two, they'll become professors or maybe go into industry. After graduation, almost all get a low-paying postdoc because it's easy and everyone expects it. After a few years of postdoc training, a minority of them manage to become competitive for tenure-track jobs, and if their spouses' careers allow it, they move to whatever part of the country has an opening. The remaining postdocs keep on going, hopping from postdoc to postdoc indefinitely. Eventually they realize they're not going to get academic jobs and they've been in academia too long to be competitive for industry jobs. And at that point they're in their late 30s or 40s, with no career prospects and no savings. Then they start thinking about exit strategies--law school, high school teaching, accounting degrees, etc. It's not good.
You're actually more employable with your MS in biology than with your PhD, because you're still eligible for technician positions in both industry and academia. On the Science Magazine career forum, it's not uncommon for people to discuss whether it would be ethical to leave their PhDs off their resumes so they can get MS-level jobs. Seriously, PhD- level biologists are a bitter and underemployed lot.
I'm not saying that a JD always works out and a PhD never does--that's obviously not true. But don't fall into the trap of thinking that no debt = no risk. There's a risk in devoting your prime working years to training for a career path that may not have a future.
Here's a great article about humanities PhDs that largely applies to scientists too: http://www.villagevoice.com/news/0417,kamenetz,53011,1.html
Here is an article naming research science as one of the careers with the highest ratios of training to pay: http://money.cnn.com/2005/08/15/pf/training_pay/