if the herbs are a major part of the beer and individually noticeable, I'd go with 21A. I think of 16E as the Belgian version of 23 - where it doesn't fit into just one other category. Like combining chocolate/fruit/oak aging on a stout, having all of the above noticeable, and moving it to 23. Note that many people disagree with me here and point to the guidelines. "Note that many spice-based Belgian specialties may be entered in Category 16E." - to me the key words are Belgian Specialties. If the beer was 16E without the herbs it is still 16E. But if it is a classic style base with herbs I put it in 21A, just like if it was a robust porter with herbs or a American pale ale with herbs.
of course, if the herbs just added a little to the overall spicy nature of the beer and blend in well, I might enter as a straight saison, 16C.
tl;dr: you'll get a lot of different opinions on possible 16E placement.
good luck--
--Michael