Personally I would start in a 152. Great for really getting the 'feel' and control of an airplane. Very good for your initial confidence and slightly cheaper. Once you get to cross country, I would then transfer to the 172. Adds experience, larger cabin/seat area which is helpfull for maps etc. Slightly more stable also, again helping with the cross country flying.
The extra cost is also pretty well made up due the extra 10 knots or so of speed over the longer distances you will be flying.
Just my opinion. Either way, enjoy it and good luck