Your choice, just keep in mind if you use the default settings, you will end up with only 256GB of space on the cache pool if you use the 256GB drive with the 500.
You don't give any information about how you use your server, whether you have dockers or plugins that would benefit from the speed, whether you leave info on the cache that might benefit from a pool. So, in light of the info not given, nobody can give you an informed answer.