Short answer: You'll make more money with subscriptions, but you'll reach less people. The market is not shifting one way or another. You should combine both monetization pathways.
Long answer: You should approach this in a few ways:
1. See what you competitors do, both in the medical field and also in the information space that you're in. This means if you are offering a newsletter, find other industries that use newsletters and see how they are doing it. You will find that memberships are more about access and IAP are more about content.
2. MOST IMPORTANT: gauge where the value is in your offering. If the monthly subscription is access to a wealth of medical information, you should not do a subscription because most people will purchase it the first month, then cancel because they used it like an encyclopedia. This is better for IAP.
If you are doing a constant refresh, however, with high value adds for being a "member" then it's a great idea to do a membership because they're not only buying information, they're buying access.
That's the most important rule in membership marketing.
3. Be able to clearly define your value relative to a charge each month. The biggest problem is when someone sees that they have been billed monthly and ask themselves "why do I still have this?"
A good example would be a month you watch zero Netflix movies. You see the $9 charge and wonder why you still have it. But, the month that House of Cards comes out, that $9 is a no-brainer because you watched 15 hours of TV.
If you consistently provide value, the membership will be a slam dunk.
4. Similar to #2, membership is rarely a viable revenue model on it's own. You should treat it, especially in the medical field (high value demographic) as a way to prove your value, then provide exclusive access to premium content (In-app purchase). Your conversion rates will be 10x what they would be if you tried to sell that premium content to a non-member prospect and you'll be able to convert more non-members to being subscribers bc of the price point.
If you want to see how I do it, check out Bluecloud Select http://www.bluecloudsolutions.com/bluecloud-select
Good luck! Stay healthy :)