Uninteresting research area and contacting potential supervisors

I've recently finished off my undergraduate degrees and am in the process of contacting various faculty members for MSc supervision. I'm interested in a couple of interdisciplinary areas and can't nail down my interests. Recently I have been offered supervision at a research lab which I had previously worked with. However, I don't find the research that the lab is doing all that interesting. As a result, I've decided to contact a couple of different potential supervisors (both are world-renowned/highly cited) at a different institution. One of these potential supervisors has not replied to my last email and I'm thinking about contacting the other potential supervisor. My issue is two-fold:

1) If I move to another research area, will the lead supervisor at the lab that has given me an offer be jilted and could this adversely affect me in the future? Is it bad to reject an offer if I'm not interested in the research field?

2) The two potential supervisors are both in the same faculty at the same institution. The one that I have contacted already is the Head of the research group whilst the other potential supervisor is a member of it. Both of them work quite closely (looking at their recently published papers). Would it be unethical/bad for me to contact the other potential supervisor given that the Head of the group has not replied to my last email? Should I tell the potential supervisor that I'm thinking of contacting that I have already contacted the Head of the group?