The simple answer is isolate the cold isle providing you have sufficient cold air flow.
If you contain the hot isle you need to cool the surrounding building areas, this creates non focused cooling and additional heat loss to areas not required. Eg with cold isles containment you only cool the cold isle not the surrounding walls and cabinets etc. Of course you still need to have sufficient cooling for the entire computer room and for the cooling losses thought external walls etc. The non critical areas can be cooled at a higher temp creating higher efficiency.
There is a couple of items to consider. If you don’t have enough total cooling your data room temp will rise and thus the cold supply air to the cold isle will also rise. The other item is if you cant get sufficient cold air into the cold isle you can consider hot isle containment and allow the additional air to be sucked into the cold isle from the surrounding areas, provided you still have sufficient cooling for the total computer room.