I have a Twingo GT (bought due to stupid distance to work, sold my 182 for it). I get 40+mpg all day long in it it great in that respect.
I have driven the Clio 1.2T (I had one for three weeks over Novemebr while my car was in the garage) and mine. For motorways and overtaking I thought that the clio was a smoother drive. It also felt torquier, though Im pretty sure the Twingo is that bit quicker mid range. The interior in the Clio is much nicer.
Neither car are super quick by any stretch of the imagination and in many case I get frustrated with the acceleration of the Twingo (I end up shouting at it), though mid range in 3rd-4th it sometimes surprises me. 1st is a joke its probably good for 20mph and the boost is limited (It is 980kg and 100bhp so the acceleration should be great but the 1st gear kills it). I'd much prefer the Clio box in the Twingo.
I plan on getting mine chipped once I have got an exhaust on it, though a de-cat is apparently a s**t to fit as the cat is on the back of the engine. I have a K and N filter on it and I have the return pipe on the dump valve removed so it hisses and wooshes and does the chavy turbo noises. (Only Forge and KTec do dump valves currently with the correct springs).
There are a few businesses doing maps and stuff for the car, there are a few claiming 130ish bhp but there are limits, I wouldn't trust my gearbox. 15-20 is more feasable. Simon Betty (the afore mentioned GT) owner has been looking into this more than me. There are rumours the injectors won't do much more so I have some 182 ones for when I finally get mine mapped.
To get decent power it'll be expensive, new turbo, strengthened bolts (it runs 9.5:1 ratio, not good on higher boost for those of you who don't know), New Injectors, FMC, map and possibly bring on gear box issues.
My recommendation? For a run about that does decent mpg get the twingo, for a bigger more comfortable car get the Clio. Want something fast? Then go for a year old 197 for the same cash.
Just my 2 pence...
Dan