ive still got my 172 but had a dci65 and am now using a dci100 daily.
dci100 is awesome, i took the 172 out the other day and it didnt feel fast, no feeling of torque, thats why its up for sale to go jap.
dci 100 has the bigger turbo so is less responsive low down, but drives more like a petrol compaired to my 65, it pulls to 4.5k no problem where the 65 was dying by 3.5k
personally if i was getting another it would be an 80 as your getting that same smaller turbo as the 65 but with the bigger injectors and intercooler for more power.
the 100's gearbox is love or hate, wish it was a 6 speed but its only a 5, its a bit notchy and weird but im used to it now so i quite like it around town. the jc5 172 box on the 80 is better though!
the 1 bit of advice i will give you is, they handle more on par with a 172 than they do with a 1.2 they handle phenominally, and ive never had one with good tyres, about to order 4 pe2's for my 100 though, they have heavy weighted steering like the RS clios, stiffer suspension like the RS but still very cumfy to drive.
and 80 will remap to over 100bhp & over 170lbft of torque for about £300 which imo will hastle a lot of hot hatches mid range down a backroad. yes its never going to be fast, my dci hits 4.5k at 120mph so its never really going to go any faster, and straight line its always going to have a narrow power band giving poor 0-60 times and off the line times but mid range punch down a backroad with more torque than a 172 and it feels much faster.
personally id say my 100 when kicking its nuts in down a backroad feels like a 150bhp petrol car. i bullyed a VTS saxo all the way down a backroad lastnight in mine!