How do C18's differ in selectivity?

All C18 phases are not the same when it comes to selectivity. Variations in carbon load, surface area, particle morphology, and end-capping (to name a few) can all effect the retention of your compound of interest.

One of the most critical characteristics to consider is carbon load; a higher carbon load will generally lead to more retention under reverse phase conditions. This is beneficial if additional hydrophobic selectivity is needed but may not be a benefit as more retention will lead to longer runtimes. 

There are also variation of C18 phases that can give additional polar selectivity, such as C18 phases with polar endcapping and embedding. These phases may provide for separations that are not possible or that may take much longer to achieve on a conventional TMS endcapped C18 phase.