It clearly needs a network
There are already calls for a seismic network on Mars. This would be very usefully complemented by a known impact event to calibrate everything and as a guarantee that even if Mars is tectonically dead (an unlikely but vaguely credible end-member of possibilities) then there would be one good event to measure travel times and map crustal thickness and seismic attenuation.
It would be dumb as NASA to do things in the wrong order, just because it "was cheaper".
If you can't afford the whole experiment, then don't begin it, especially with the "wrong" half...