I've owned both the Response 1SC and Tablette 50 Signature for over three years.
Generally, speaking the Tab 50 Sig is a faster, livier speaker than the 1SC. I think it images better as well. The 1SC is more settled and less likely to get ruffled during complex passages, and it has slightly better bass.
As to which is the best, I think it comes down more to which you'd prefer. A lot also has to do with your associated equipment and your taste in music and presentation.
Generally, speaking the Tab 50 Sig is a faster, livier speaker than the 1SC. I think it images better as well. The 1SC is more settled and less likely to get ruffled during complex passages, and it has slightly better bass.
As to which is the best, I think it comes down more to which you'd prefer. A lot also has to do with your associated equipment and your taste in music and presentation.