I was in a similar predicament as you a few months ago. In the end I chose spesh for the following reasons:
-spesh is marked up 12 now... even at 35 it gots to about 47
-Monash offer a bio crash course for med students who didn't do bio
workload wise I think bio needs more time devoted to it since it isn't just memorising something but apply a new concept to a situation. If you understand something in spesh then all you need to do are practice questions.
I suggest you play to your strengths, if you are alot better in bio than in spesh then choose bio, but if you think your ability is about the same in both subjects then I think spesh is ideal. 30 in spesh=42, a 30 in bio=31ish
However you should choose subjects you enjoy because you'll be more motivated.
And to answer you question about stress, you'll find that anything you're behind in will be stressful, so stay ahead!
Hope that helps abit
