Which one is stronger: hydromorphone hcl 8mg or oxycodone hcl 30mg?

Neither: those are close to what most tables would consider equipotency, where hydromorphone is about 3 times more potent than oxycodone. However, there is a huge amount of individual variation in what the equivalent potencies are in different people, as well as in the potential side effects of the different opioids. You should be asking your physician or pharmacist these questions.