Reload cache does just that, clears the current data and reloads the nfo data from the tvcache.xml file. This procedure is very fast.
Rebuild tv show deletes the cache and rebuilds it by reading each nfo, the nfo files are not written to.
Early versions of MC had a great many problems with keeping the cache file in sync with all changes, especially with movies. Newer builds should have most of these errors ironed out so neither of these options should need to be used very often now, if at all.
Over the next month or so I will be writing a manual for MC, with full details of all functionality and useage. This is long overdue.