Replying to Upgrading from t3i

So I have had the canon t3i and I am thinking of upgrading to the canon 7d. Is it even worth the extra money because the specifications are very similar. For example the sensor size is practically identical. I shoot mostly video for skiing but also landscape. Should I upgrade to something like the the 7d or 7d mark II? Are there other models that in the similar price range of the 7d or maybe in full frame from canon? Also another question, if you use a crop sensor or aps-c lens on a full frame camera will it not work or be compatible?

Thanks and +k for explanation