I fly a Seneca II and an Aztec (F).
The Seneca gives an honest 165 Kts at 65% and burn just under 23 gph.
The Aztec gives and honest 160 Kts at 65% and burns just under 30 gph.
Both are great airplanes.
We just put on LoPresti wing tips on the Seneca II and have not had a
chance to see how much increase in speed. Another Seneca II I have
flown has both the wing tip and cowling mods. It does 175 kts.
Seneca II has counter rotating propellers which helps in single engine
operations.
Seneca II has 200 hp/ engine. Aztec has 250 hp/ engine.
The aft baggage compartment in the Aztec is much easier to load than
the Seneca.
Either the Seneca or Aztec will make you a very nice twin. Also
concider the twin Commanche.