Freddie (with a range of F2 to F6!)had a superior range to bono (who has a range of E2 to G#5). Bono did (does?) Have an excellent range, but not quite as expansive as Mercurys was at his peak. But of course range is not everything, and the quality of ones voice is far more important. I tend to think that bono has a slightly better voice that Freddie, just because of bonos emotion, passion and desperation that his voice conveys. Bono has one of the most effective and expressive voices I have ever heard. Think of the bitterness and regret in "one", the desperate and determined " wide awake" of Bad, the yearning and hope in "where the streets have no name". Freddie's voice was amazing also, (think of Barcelona, how can I go on, don't stop me now, and the legendary " prophet's song". As songwriters, I find bono superior. I know this sounds pretentious, but bonos lyrics seem more "mature" than Freddie's. His songwriting can be very hazy and impressionistic (TUF), it can have a narrative appeal (ISHFWILF), and have a beautifully poetic nature as well (OTH). I find Freddie's writing to be good, but not particularly resonant.
I must say, Freddie had an amazing knack for melodies and catchy vocal hooks. There are so many catchy and unique sounding queen songs. So for creating a memorable melody, I must give the edge to Mercury.
As performers, it is hard to decide. Freddie was larger than life, eccentric and energetic. Bono is those things, but to a lesser degree. I find bonos live performances to create a genuine emotional connection with the audience, and he seems to go to a spiritual place for some songs. As a classic frontman, Freddie wins. Its impossible to definitively say, but I slightly prefer Bono.