A 3g battery on iphone 3gs is not compatible because create error 29 but a 3gs battery on iphone 3g is the best thing.
No problem in many years,i fix iphone and i've fixed about 400 and replaced more than 60 battery and never problem with this method.
The big problem is to find a good battery (truly original).
The best type of battery in iphone are the white model with apn:
616-0391 for iphone 3g