Possible Duplicate:
When is it appropriate to use 'that' as opposed to 'which'?
Sentence 1:
The motorcycles with designs which involve an engine, a shaft and a gearbox, form a significant subset of the product line of BMW.
Sentence 2:
The motorcycles with designs that involve an engine, a shaft and a gearbox, form a significant subset of the product line of BMW.
Which is more appropriate? And in general, is there a rule of thumb to decide between using "which" and "that"?
