If you like the lab you're in already then I'm not sure why you wouldn't want to continue in it. If you are unsure about a career in basic science, I'd wait on applying to combined programs until you are sure.
In my opinion, the training of a PhD is very valuable for someone doing basic research, but realize that it's exceedingly difficult to succeed at that even by itself, much less while doing clinical work as well. I know everyone thinks they will be the special unique snowflake that can do it all, but most will have to pick.