i am also looking to add tower speakers..though new to boat audio (i have done a lot of car audio systems)
go with a horn based system- though they cost a little more- you will be able to hear them....i would predict within the next 2 years- all tower speakers (mid to high end) will all be horn based.
i am pretty familiar with image dynamic speakers(both subs and seperates) and they state of the art (they make design and produce their own drivers- the id camealon seperates replaced my dynaudio gt240 seperates- they are that good)
it is nice to see a fresh ground up design specifically for wakeboarding...though they may seem pricey, but it is a pretty good deal if you were to make something custom.
though i havent heard nvs yet -waiting for big ed to put them on his x-2(before summer would be nice-
--just kidding big ed !!!) a lot of boat owners really like them. and i havent heard anything bad about them).
wether one is better than the other, i dont know. however you wont go wrong with either one...
good luck !!!!