I got a 16 for my 20x 18 cube. I went with 3 12in AI sol for the 48. My fear was good coverage. on my cube the stock sb lenses didnt spread the light as evenly as I wanted and didn't hit all the sides without having it up pretty high.
The best possible coverage for a tank is a t5 array or MH with large reflectors, right?. So (as im in lighting professionally) how is a 32in fixture going to cover 48in. it would have to be up really high. (think flash light) move the flash light away from the subject the beam is wider and covers more area. a lamp with lenses focuses the beam even more. so farther away.
By pulling lenses it makes the beam wider so it can be closer BUT, its still a 32in fixture on a 48 area . Buy using 2 16 in fixtures you have a wider gap between them to cover more area.
I like the approach of the Photon V2 in this respect or the big Maxspects. . And I do wish SB would do something similar. and yea 3 16in fixtures would be perfect But...... $$$
If you look at a good radion array this is the same approach.
heres one that validated my approach after I had done it.
https://www.reef2reef.com/ams/dr-joshi’s-500-gallon-sps-dominated-mixed-reef.2/
hes using all wide lenses and has multiple arrays to cover and overlap the light. this reduces hotspots AND shadowing by hitting corals from every angle. (keep in mind this one is HUGE)
And hes a legend in reefing.
the way I added it up the 32 was as much as 2 16 any way. On the 48in tank a got a SWEET DEAL on the 3 ai sol's. so thats what sealed it for me. ($3000 in used lights for less than one new sb? hmmmm...Ok!) and I still pulled half the lenses so I could use my canopy. I hate seeing the light in my face. (i do that at work

)
People love thier tanks and their lights. Do google image searches and youll see what I mean about coverage. IE The Single Kessil look.