Stacking means backplanes are supposed to be conected by an external link allowing the hardware to behave as if htere is only one swich. From that point of view, there is only one management entity, only one STP bridge id on the network, one CAM table, etc.

Ethernet bundling means the switches are conected to each other as standalone entities by standard interfaces (as many as you like, but stardar interfaces after all). That means there are two separate management entities, two STP bridge id, two CAM tables, etc.

The expected performance of etherchanneled switches is much lower that what could be achieved with stacking technologies. Of course, switched with etherchannels are much cheaper that their stacked counterparts.