There are lots of mechanical differences between the 2 engines. It isn't just the Map that makes the difference between 140 and 170. The brakes are bigger on the 170 too and it will also come with speed sensitive steering. I know that a 140 can be mapped to about 175-180 but the brakes may not be up to coping with that power.
The 170 in the Exeo is not the same engine as the old PD170 in the
MK2 Leon FR, it does not suffer all the DPF problems.
Go for a 170 if you can, they go better, they only use a sip mire fuel, they cost the same to tax and they cost only a few quid extra to insure. Also I guess the 170 will be slightly easier to get rid of as it's the most powerful diesel one.
BTW to best bit about the 170 is the all red TDI
badge on the wings, none of this silver "T" and red "DI" going on with the proper derv engine
