I'd say physics, but you have to realize that there is little prospect for gainful employment unless you stay within academia or find your way into a national lab. Either way, you have to go all the way to Ph. D. in order to even be considered for the jobs available. In most cases, you'll probably need some post-doc work as well. That environment can be rather brutal. The "publish or perish" workplace was too stressful for me, and I decided I had to get out. I have only rarely regretted the decision, but as a mech. engineer, I had that luxury (plenty of jobs outside of academia). That isn't nearly so true for physics, sadly.