Bluewave

1) More precise

2) Can be adjusted under tension with no additional tools

3) Can be adjusted without removing a shroud pin (safer on water)

4) Harder to accidently remove (never actually had this happen to be)

A two row Vernier is enough precision on shrouds and caps for me. But a bottle screw would be nice on lowers where smaller adjustments make greater changes to tension. However, this could be achieved with a regular bottle screw. 

2 and 3 are only really plus points if you want to be changing settings between races. Good when I was sailing 29ers and 49er and we'd have 4/5 race days with 10 hours on the water. But in amateur stuff with 2/3 race days and 3/4 hour days I am happy to set and forget. 

Point 4 I am not sure is a point. 

So overall they are a lot of extra expense for not much benefit. I would like bottle screws for lowers though, but a cheap set without in built spammers would suffice.