There are quite a bit of differences between Sencha Touch 1 and 2. Performance for Sencha Touch 2 is a lot better than Sencha Touch 1. The class system is a lot more mature and inline with Ext JS 4. MVC implementation in Sencha Touch 2 is (and once complete) will be far greater than Sencha Touch 1. So Sencha Touch 2 is a better framework. The bad part is we are only in developer preview stage so if you need to have an application done in the next month then it may be a wiser idea to stick with Sencha Touch 1. I personally have dropped Sencha Touch 1 for Sencha Touch 2 for my projects. Yes there are bugs but I either create overrides to fix the problem or work on some other piece. PR3 is due out soon and has a lot of bug fixes that are crucial compared to PR2.
Like Mitchell said, if your project's deadline isn't in a month I would start with 2.0. In addition to what he said, learning 2.0 will give a big jump on Ext JS 4 if you're not already using it as the MVC and class system is very similar. I think many of the Ext JS 4 guides also are good for Touch 2 - MVC, classes, and theming using SASS.
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