I've flown both. If you have an IR, go for the Seneca, use its turbochargers to get you in to the FL teens of Airways flying (with oxygen of course) and avoid Eurocontrol en-route IFR charges that the Aztec would incur you (most Senecas have a max AUW of 1999kg).
The Aztec is a tougher beast that will be more forgiving of poor engine management. It is also more suited to grass strip work.
The Seneca is roomier if you want to fill the seats but you will have to trade a lot of fuel in lieu.