Better? Hmm, I dunno, I would call it a draw, Mina aside.
Not the biggest fan and talent wise I don't see much between him and Keane, but......
One big difference is that Tarkowski is a leader/organizer type who takes charge and doesn't seem to have confidence made of tissue paper. Mina aside that's what we have a total absence of throughout the centre of the park.
With a 'youngish' set of defenders and a new who are prone to mistakes, then can see the sense in him being a target.
Tarkowski on 90k p/w, Keane leaving for a fee (say 10m) think Keane's on a bigger wage than 90k as well, and pretty much no difference in age (plus Tarkowski won't have the Keane baggage of all those horrid games for us)
In isolation it'd be a deal that made good financial sense and probably sense in terms of overall strengthening the position.