T5p's sound fantastic, theres a lot that can be searched about them. I would rate them as neutral to slightly lighter on the bass than real life (judged upon classical, symphony/concerto type stuff), clarity, feeling of space, and separation are superb.
ultrasone sig pro: they are bassier than the t5p's, I would rate them slightly heavier on the bass than real life. the clarity, separation, feeling of space aren't as good. But they sound warmer and more pleasant, I wouldn't necessarily say they are more accurate though.
as far as isolation goes, they're both pretty good. I think the sig pro's might isolate a tiny bit better, because of the higher clamping force. The t5p's are significantly more comfortable, but that said, the sig pro's aren't uncomfortable (unless you have large-ish ears, then I can see that your ear will likely rub against the cloth covering the driver.)
the sig pro's are smaller by a good bit, and foldable which makes them a lot more portable if thats an issue.
overall i think the t5p's are a slightly higher quality headphone, but it depends what you're looking for.