In all honesty, don't look at admissions rates as they are not predictive of your admission chances. If you have the scores that are in the top ranges (31+), 3.9+ GPA, and all the other academic stuff, I would say that already increases your chances to 15-20% at lets say Harvard. After that, your essays and EC's will carry you. If you did something amazing, you can expect chances to rise to 70% and if everything looks good, it is really how you compare to others. That said, I would say that bs/md and Ivy league has a lot of cross admits and if you can get into one, you should certainly be able to get into the other.
So basically,
Amazing academics + ECs that set you REALLY set u apart + good everything else = admit unless random badluck plays against u.
Amazing academics + Above average ECs + good everything else = maybe + luck
Amazing academics + avg ECs = reject
Bad academics + Amazing ECs = reject