I don't know if it is possible to give a precise consideration on which package is more powerful (TikZ or the tkz bundle): what is true is that they have a different syntax, but I think there are cases in which TikZ provides a simpler programming solution and others when tkz is more straightforward.
For example, the macro
\tkzMarkAngle, is one of the latter case. You could find the code of the macro typing in the shell:
gedit $(kpsewhich tkz-obj-angles.tex)
The following document tries to replicate the same picture with both packages and with a similar programming structure:
% to display picture and code
% equivalent: \tkzDrawVectors(O,A O,B)
\coordinate (O) at (0,0);
\coordinate (A) at (2,2);
\coordinate (B) at (-2,3);
(O)--(x-b) arc (125:38:0.95cm) -- (O);
As you can see, the second picture is not precise as the first one, because inside the definition of
\tkzMarkAngle there is the exact computation for that. Not that you couldn't do the calculus with pure TikZ, but you need to complicate the code.