Id say speed.

Lets say instead of viewing rivers and such... we put it in vacuum, and
in space (like all good theoretical physics questions..) We take two
blobs of water, one with 2x the volume (and therefore mass) and one
with 2x the speed (or v.. same thing if same direction) answer is
simple. 1/2mv^2. v^2 is a second order term, and therefore increases
faster than m.

(this assumes that Ke is the only limiting factor ... which Im guessing
is pretty close.. so "all else being equal, a higher velocity stream
will be more destrictive" )

Yes glaciers can erode mountains, but they're order of magnitudes more
massive.. give me a waterjet of that velocity, and i'll cut you your
mountain.