I'll have to say Empire as well.
The Empire has a long tradition of fostering cooperation between the races and respecting other cultures, the Stormcloaks on the other hand seem really racist and xenophobic. On that note alone I think the Empire wins out. Every culture in Tamriel has a lot to offer, and recognizing that is what makes the empire such an enduring force.
Yes, the empire got more or less beaten by the Thalmor and had to accept an extremely unfavorable treaty, but as it turns out, the whole Talos thing is not only specifically about breaking up the Empire and the Nords, but they also set up Ulfric to start the rebellion. The Stormcloaks are ultimately much more the Thalmor's puppets than the Empire, which is desperately trying to preserve it's fighting strength for the next conflict, while the Stormcloaks are happily letting themselves be divided and conquered.
There is also the fact that the Empire is the creation of Tiber Septim, aka. Talos, so if you are a genuine Talos worshipper it's kind of questionable why you'd want to break up his legacy and greatest accomplishment. Yes, the Septim line is dead, but the Empire is still the most tangible manifestation of what Talos actually wants, because he created it.