There are a lot of criteria to use when choosing an eCommerce framework. There are a lot of choices and each has its advantages. Hopefully your question will get responses from developers who have used all of these technologies.
My background includes years of development with ATG and I am one am one of the developers on Broadleaf Commerce
so I can provide some perspective on these two options ...
ATG and Websphere Commerce are the clear market leaders in the enterprise commercial e-commerce offerings. Both are feature rich and provide very nice business users tools. I personally think the ATG business tools are better and the solution offering is more complete than WS Commerce. As a developer using these platforms, you are likely to experience some frustration due to the technology stack. ATG for example is highly extensible but uses a proprietary component model, persistence technology (repositories), and presentation layer (form handlers/droplets). I have not had as much experience with Websphere but in my opinion it suffers from its EJB2 based component model. As a general rule as business requirements differ from the OOB implementations the work can get exponentially harder with these frameworks versus similar open source stacks. This is in large part due to the fact that you don't have the source.
is an open source eCommerce framework that is built on top of leading open source technologies (ie. Spring, JPA (hibernate), ActiveMQ, Lucene ...). Like other open source alternatives, you likely will need to build some features that would be out of the box in a commercial offering. Integration tasks using this stack will generally be easier and you should never find yourself coding against a "black-box". Broadleaf Commerce (BLC) is currently running The Container Store
but it is relatively early in its development cycle. The key technical advantage of BLC is that it is a well thought out, extensible solution built on technologies that most of the Java community are familiar with.
Hope this helps,
-- Brian Polster