Here's a summary of each of the services...
1) The Marvel Unlimited library includes over 17,000 comics!
2) You can save up to 12 comics to your device for offline reading.
3) The Marvel Unlimited app is available for Android and iOS, and subscribers can also read comics on the Marvel website.
4) A monthly subscription costs only $9.99, or an annual subscription is even better value at $69. Look out also for special introductory offers from time to time, such as your first month free, etc.
5) The Marvel Unlimited library is fairly comprehensive from 2000 onwards.
6) New comics are added every week with recent titles added after a six month delay. The exact number of new additions varies from week to week, but 25-40 per week is fairly typical. As well as recent (six month old) comics being added, each week you'll generally see a few comics from years-gone-by added as well.