The Seneca can be deiced, has turbos and a more modern cabin. Also the single engine climb is better (all things being equal). I m not sure if the Aztec has counter rotating props. The Seneca has.
I believe the load capacity of the Aztec is quite good.
Seneca I and II are not my thing, III and IV are. The IV is basically a Seneca III with the looks of a Seneca V (round air inlets and the light color panel).
The V has different engines which are alleged to be a few knots slower.
Both machines are intended for longer trips where altitude capability (turbos) and de-ice is almost a necessity