phpBB Native Fulltext with 0% common word threshold is perfectly acceptable for a small board. However, watch the size of the search index as your board grows larger.
mySQL fulltext is arguably a better option for a medium sized forum, both in terms of search index size and the fact you can do phrase searching (this is not possible with phpBB Native Fulltext).
Anything over a million posts and you'd be silly not to use Sphinx.
To give you an idea of the differences, I have done quite a bit of experimentation on this in the last 6 months. Here are my observations based on my board (~1.9 million posts, although has been well over 2 million in the past):
* phpBB Native Fulltext search index was 3.5-4GB alone, over 50% of the total forum database size. This is with a common word threshold of 5%. In a test environment it would take over 24 hours to rebuild the search index. In a live environment this would take much much longer. In fact, we first moved from phpBB2 to 3 and this search engine was first introduced, it took 2 weeks to build the search index.
Search queries with large numbers of results would take over 30 seconds. Many of the most important keywords on our forums were counted witihn the 5% threshold and so the search function became largely useless over time.
* mySQL Fulltext had much better granularity as a search backend. You could actually find the posts you were after and you can search for phrases. Search index size was also much much better at around 1.5GB and only took a few hours to rebuild. However, the search index was still stuck with a minimum word length of 3 characters. More complex search also took longer than phpBB native - well over 30 seconds, so it is not suitable for deployment on a large board
* SphinxSearch - with a minimum character length of 1 (ie every single word indexed) and infinite inline wildcards, the search index is under 1GB. It takes less than 10 minutes to rebuild from nothing. In fact it is scheduled to completely rebuild the index every day at around 2am when the board is quiet. There is a delta index running every 5 minutes to update search results as soon as they are posted. This delta index takes less than 10ms to build. Searches with extraordinary complexity all take less than 100ms. Most searches are done in a few thousands of a second. I can do phrase searching, proximity searching, quorum searching, exact searching, searching for grammatical equivalents (lemmatization) and custom mapping of search terms. Sphinx for the win....assuming your hosting allows you to install on your server and you are prepared for the learning curve to set it up.