What are some alternatives?
When comparing Mobile Sketchpad and Axure RP, you can also consider the following products
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
Invision - Prototyping and collaboration for design teams
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.