for online shoping cart which tool best ?

which one tool is best for online cart web application ?
ATG or rails(like Agile Webdevelopment has a sample cart application)

Not sure what you mean when you say what tool is best BUT Here is two
routes to take:

Beginning Ruby on Rails E-Commerce: From Novice to Professional
(http://www.apress.com/book/view/1590597362)

Its old now BUT it is a VERY good start and will get you going from
the cart to checkout.

OR us use this:

http://code.google.com/p/substruct/