Dentistry or Engineering?

I'm a graduate student in structural engineering at Stanford. I work at a consulting firm, designing buildings, bridges, towers etc. I work 50 hours a week on average and salaried at 58k. My work is alot of fun, but it's quite stressful, the pay is marginal, and it has no job security especially enduring this recession. I been thinking about switching to medical field, specifically dentistry. I will need to spend 2 years on prep cles, 3-4 years in dental school, and take out 400k worth of student loan plus around 250k of opportunity cost if I'm not working. I'll be 30 by the time I'm done, is half a million dollars and 5-6 years worth of education worth it? any advise is appreciated.