Anywhere in China that you may choose to live, you can learn Mandarin.  Mostly because besides being the official language of China, it is the one taught at all schools within mainland China, so most of the youth speak Mandarin.  Cantonese is mostly spoken in and near Hong Kong.  any other of the 230+ dialects are mostly spoken at a particualr village, or at home by residents.
So, without any fears, you may study Mandarin in any of the larger cities, even second and third tiers.

Another consideration about Beijing, besides cost of living (which is quite high) and the fact you are familiar and have friends already at Chengdu, would be the air pollution, which is quite bad in ABeijing.

In your shoes, I will go to Chengdu without a second thought.

Beijing will give you the more standard accent and you do not have to worry about lack of practice out of the classroom since most here speak Mandarin. I lived in guilin for four years but since moving to beijing last august my chinese has improved  more in 5 months than it did in a full year. Guilin and Chengdu have their dialects amd some residents have fun tripping up mandarin-speaking foreigners by using their won dialects instead.

As GuilinRaf said, Beijing will give you the most standart Mandarin you can hope for (with the extra 儿's in almost every word). But, unless you're planning to concentrate on learning and NOTHING ELSE, you'll join the ranks of the people who either think "Beijing isn't what it used to be" or "I've been here a few months and I already don't like it", and enjoy the global depression that sets in during winter!

the place is not that important,but you have to open you mouth,and know more and more friends with can speak chinese..

any where but if you have the skells to gain some thing and have  intrest in study than you can learn chinese from any small school in china ,so this is not some awosome question.you should have to decide by urself bro. i think so