The 140 is a great car. If you drive them both you'll want a 170. If you only drive a 140, you'll be happy. If you're bored after a while then re-map it. We've had a 140 in with 180ps (claimed) after a re-map and everyone who drove it preferred it to a 170.
From my dealership the sales split is literally about 50/50. So that doesn't really help.
Re-sale wise, you'll get a bit more for a 170 after a few years but you'll pay more to start so it really comes down to how much power you want! (I guess that's fairly obvious though...

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If it were me and my money, i'd go for a 170 then remap that after a while when i'm used to it......
NC