Its not difficult to go from C152 to C172 after you have passed your test, so I shouldn't let that worry you.
My advice (assuming you are male, reverse if you are female or inclined in another way!) is if you have a nice lady instructor, go for the C152, as you'll be jammed in nice and close. If you have a big male instructor then I would recommend the 172. I started learning in the 152, I'm 6'1" and so was my instructor, and after 15 hours of him putting his arm around me, we decided it'd be best to switch to the 172.....which is a far nicer aeroplane to fly I might add.
Cheers
EA
