This is size comparison of GW turret base (coming with Imperial Bastion set) with a turret base produced by Old Crow Models:
GW product you have as a bonus stuff "for free". The other one cost much more (but will not make a hole in your pocket) but is also bigger and more sophisticated.
This is how do they look with their respective turrets (sold separately in case of Old Crow):
That's all, I'm sure both will find some proper use.