I own both of them. The Renegades are nice, the Great Wars are marvelous.
Renegades are just a little bit chunkier, bigger and less detailed, but if you see them on a tabletop you wouldn't notice unless told.
The IMO most important thing with mixing germans apart from the size are the helmets, and they aren't that different.
On the other hand, brigade miniatures does some nice germans, but their steel helmets are completely different from Renegade ones (the helmets rim).