Choose the FTO first, then worry about the plane... I have never flown the Seneca but have been told repeatedly that it can be tricky at times. The Duchess and the Seminole
are much better training machines with vice-free handling and a number of easy features (like contra-rotating props, landing at MTOM, VLO and VLE=cruise speed on the Piper, plus a layout that looks very much like the Arrow). Altogether, I guess you can't go wrong with either the BE76 or the PA44-180.
Soon the glass-cockpit Diamond TwinStar may be considered, but I hear its delivery is still some months in the future
Cheers