Don't get a petrol cooper they are crap on fuel, worse than the GTI! And much slower, we had one for a year... It replaced the town car yaris tsport which was quicker and MuCH better on fuel. Cooper S is a hoot but probably worse again on juice.
The 140 is a smaller turbo but less lag iirc. Can't stand laggy motors... Had a Volvo Eco same as focus 1.6d for a while and was constantly having to find lower gears as there was no pull at low RPM, the GTI incomparison pulls well from almost nothing, K03, which is where you would think the diesel would be better... Longer gearing for the economy no doubt.
Go do some test drives and see what you think.