I've had all the engine types a 156 has to offer over the last 4 years or so, from a 1.8 twin spark, to GTA and now hunting down a good diesel.
honestly if you can find one the 2.4jtdms offer the best all round package.
the 4 cylinder petrols are nice gutsy motors for what they are but an n/a 4 pot will always lack ultimate tourque. twin sparks you are looking at 155bhp from a 2.0 and I think the JTS has 165bhp but you are looking at 152ft/lb or tourque - which is what counts when you are rolling down the road!
if you can afford to run a V6 then please please please buy a GTA. I've just sold my GTA due to being made redundant and its honestly the hardest thing I've ever had to do. it maybe a bit of a money monster but the moment you plant the loud pedal none of that matters!
the best between power and economy though has got the be the 2.4jtdm. a 4 cylinder engine can never be smooth as you can't get a harmonic firing sequence out of a 4 pot. the 2.4 however is a 5 cylinder motor, so its smooth as silk, you wouldn't know it was a diesel and it even sounds good!
they are nice and strong at 175bhp and 284ft/lbs of tourque, nearly twice as much tourque of the twin sparks, which is what you will notice. I think the 2.5 V6s are about 195bhp so not far behind them and still returns about 50mpg.
a 2.4jtdm I test drove over the weekend said as I was cycling through the computer that I was doing 54.6mpg average. my GTA used to do 24.8mpg average