Difference between difference between bc547 and 2n2222
There is not much difference when it comes to electrical properties of the two transistors. You can have a look at the datasheets :


P2N2222A
BC546/547/548/549/550


The two major differences that I find are:


the power dissipation (a measure of power that transistor can provide):

2N2222A - 625mW

BC547 - 500mW
collector current (output current in common emitter config):

2N2222A - 600mA

BC547 - 100mA


So your application defines your selection in such cases. If you need more current amplification for same biasing voltages you might want to go for 2A2222N. 

Read the datasheets carefully and make your choice.