The DC25 uses the Dyson ball technology... it's system pivots on ball instead of wheels that grants you a better handle when moving the vacuum. It also comes with more accessories.
The DC28 does not have the Dyson Ball technology and it pivots on wheels. It's cord is slightly longer. It is using the Airmuscle technology that will give you a better succion for homes with carpet. These are the main differences.
Hope that helps
