There's a lot of comparisons between the 2, and Jessie J is often called "the British Katy Perry".
But Jessie J, technically speaking, has the better vocals.
Do you think Katy could sing Jessie's songs and vice versa?
Katy is to Jessie as Rihanna is to Rita, like the latter in each comparison is British and technically the better singer but is also just kind of a copy of the formers.
Kay could but not as well. I would enjoy them more though, and they'd be more successful