As regards invertebrates, they also hold the studbook for Fregate Island Beetle (Polposipus herculeanus) and Gooty Sapphire Ornamental Spider (Poecilotheria metallica), which are VU and CR respectively.
One of the factors which has swayed me for the present - although I still look forward to arguments being made on either side - is the fact that much of the ectotherm-specific side of exhibitry at ZSL is in the same depressing state of slow decline as is the case for much of the rest of the two collections in question, with the now-closed Aquarium at London having been left to slowly stagnate and fade into oblivion over the last decade or two, and the Reptile House sometimes feeling like it is going in the same direction, with more duplicated exhibits or boarded-up tanks each time I have visited. The Invertebrate House at London is rather better, although I have not seen it since some of the old aquarium collection was relocated there so cannot attest to how this has affected the whole.
It has been some time since I visited Whipsnade, but the reptile house there in the "Discovery Centre" has long-since been closed to the public and was in a state of some decline before closure - although I think it may be re-opening, or recently has done so, in significantly reduced form as another overflow of the former Regent's Park aquarium collection.
Conversely the standards of exhibitry for endothermic species at Bristol are pretty solid across the board, with the invertebrate house, reptile house and the stand-alone frog breeding pod particular highlights - and all of them seem to be developing and improving, rather than declining as the ZSL ones are.