We refer to Confluence 5.9 and the projectdoc Toolbox 1.10 for our discussion.
Please bear in mind that projectdoc is our product - so there we are certainly biased in favour for transclusion.
Nonetheless we hope that our discussion is fair.
If you think this is not the case, especially if you feel we are lacking knowledge of features in favour of the excerpt feature, please give us a shout! We will check and adjust your issue with our presentation.
Summary
The excerpt feature is easy to use, but the configuration options are quite limited. Transclusions, as provided by the projectdoc Toolbox, are very feature-rich, but may be difficult to employ for new users.
If excerpts are providing all you'll ever ask for, you should stay with it: it is free and is part of every Confluence installation.
If you are asking for more and paying for additional features that make you more productive is not a problem for you, check out what transclusions have to offer.
The following table lists features that are related to one of the macros discussed, but are implemented by different macros. In this case the macro providing the feature is listed. No extra cost is involved to use the mentioned macros, since all these macros are either part of Confluence or the projectdoc Toolbox Add-on.