I have been supporting Blackberry's for many many years, I have also Supported Good's Solution and am beginning to Support the Active Sync solution also.
Blackberry is true push and utilise's the Blackberry devices unique id or PIN to associate and send things to the devices, but none the less the server is doing the work with "Notify" messages from the exchange telling it when things change. Most Blackberry users will tell you,there email is always on the device a few minutes earlier than they hit the outlook client.
Good - was more or less an offshoot on Blackberry from my understanding some of the core members of Good's Design team were ex-RIM employee's so i'm guessing RIM are getting a nice settlement out of each Good subscription. Good again is a true push technology however and have some very good tools to troubleshoot and check if a device is still alive on the network.
I personally don't regard this as a push service, to me it seems like an outlook client sitting on a mobile phone, nothing more nothing less. I did however believe there was an MS push service but I tend to believe this is just a coded SMS/TXT? which is sent to the phone telling it to go grab something new..
So overall yes there is a significant difference in the solutions and also with the devices which can be used for any given solution which ultimately means each business will need to choose the best for them.
thats my 2cents anyway