I have played both of them.
It used to mis-print 16 as 15L.
As I recall, if it says "Ball Feel 15L", that is 16g.
"Longevity 15L" is real 15L.
Tonic+ is cheaper than VS because of gauge inconsistency anyway.
If you use dense string pattern(18x20) and hit kinda flat, you might not feel
much diffs. If you hit topspin a lot, you may want 16g rather than 15L.
and 15L will feel slightly stiffer than 16g.