Some people have suggested that the almost complete overlap obtained with 2x250 V4 is a good strategy because it gives the best error correction. I disagree. In my opinion, for most applications the reduction in error rate is not dramatic and does not compensate for the improved phylogenetic resolution you get with longer sequences.
Using UPARSE or
UNOISE gives very accurate OTUs regardless of
the overlap length. You can even get good OTUs if you throw away the forward
reads and use reverse (R2) reads only. I therefore recommend sequencing the
longest possible amplicon that will give a minimum overlap of around 20 to
30 nt.