What's interesting about "reach" though is that it seems not correlated with influence, as in the test done by Christakis and Fowler http://edition.cnn.com/2010/OPINION/11/16/fowler.christakis.... So maybe your 1st tweet had much more "impact" (>number of retweets could already indicate that btw) even if much lower reach.
A 2cts idea on impact ('did it resonate with your crowd or cross the board'), staying on twitter (vs. if people did sthg you recommended to do like in their study 'buying a book') could be to see the n degrees of cascade retweets. Ie. if your direct follower retweet that's "level 1 retweets" if some of their own follower retweet "level 2 retweets" etc. to see if there is a cascade effect or not.
A 2cts idea on impact ('did it resonate with your crowd or cross the board'), staying on twitter (vs. if people did sthg you recommended to do like in their study 'buying a book') could be to see the n degrees of cascade retweets. Ie. if your direct follower retweet that's "level 1 retweets" if some of their own follower retweet "level 2 retweets" etc. to see if there is a cascade effect or not.