Which of these is better and what are the differences?

I see the explanation here:

Search Type
Fulltext searching uses a search index that is constructed by MySQL itself, whereas vBulletin's own search feature uses its own index.

Delete Indices
When switching a forum to the fulltext search mode, you will want to consider emptying the indices that the default search engine built. These indices are not used by the fulltext search and consume a large portion of your database. You should be certain that you are going to permanently use the fulltext search before removing these indices since, generally, it takes a lot of time and server load to rebuild these indices. Another consideration is during any time that the fulltext option is enabled, these indices will not be updated by any new posts. Using fulltext search for an extended period of time will leave these indices stale and you may still wish to rebuild them.

You can also empty these indices in the Update Counters section of Maintenance.

You may want to optimize the postindex and word tables afterwards by going to the Repair / Optimize Tables section of Maintenance.
Ole dumb hillbilly me... this doesn't tell me a whole lot.

Thanks!