I was faced with this dillema a few years ago when I bought mine and test drove them both. Basically despite what you will read by some they are the same car with one about 80kg lighter being striped of some creature comforts.
Its basically down to personal prefrence and whether you can live without some comforts. I decided to opt for my comforts but I must say I do like the blue that the Cup came in and would have picked it were it an option on the 172 ,but the interior was crap and I couldnt live with it even if it were about two grand cheaper.
After all is said and done drive them both and decide what suits your needs and not that of others.
There is a thread on here (infact theres loads) that lists all the differences, do a search.