The difference is that we know exactly what bacteria are in One and Only and there is research available that validates their claims. I've used it myself with success. On the other hand Tetra are not being open about what their Safe Start product contains, although we know it had similar origins to One & Only, so we can't tell if it is effective or not.
In either case the TSS instructions are different to One and Only, whichever one you use be sure to follow the product instructions precisely - otherwise it's your fault if it doesn't work
It's best to use the instructions on the websites as the bottles don't tell you enough.