There is absolutely no problem with discussions about other carts, so long as the discussion is in the context of us here at osCommerce.
For example, there would be absolutely no problem in someone asking;
At the moment I am using "whatever-cart", I'm thinking about changing to osc...does osc have XYZ feature?
And no problem in someone replying;
It doesn't have XYZ, but if you install osc and then download this addon...you will then have something very like XYZ
There is a problem in that person given advice such as;
Keep using "whatever-cart"
It's a matter of common sense. Does the reply that is being made "help the situation" with regards to osC gaining positivity. If yes: post it. If no: dont post it.