Both sencha and jquery mobile are good mobile web frameworks and are popular for creating mobile websites, blogs and custom mobile apps. If you need a high level comparison of the two frameworks here it is :
Sencha Touch
- Higher learning curve
- More robust than jquery mobile
- Documentation does not cover the power of the sencha framework
JQuery Mobile
- Easier to learn and experiment
- Slow transitions and effects
- Not as feature rich as sencha
For a more complete discussion follow this thread.