Diana is my favourite act of all time.
Her auditions were spot on, and I actually think the song choices for her were great with the exception of week eight. She nailed it week 1 - 4, but obviously fell apart once she got ill.
There were obvious problems with Yellow and Patience in week six and seven, but they were still decent performances and she still managed to outperform Daniel, Eoghan and arguably Rachel at that point who just wasn't delivering/awful song choices.
Week eight was a terrible week for her, crap song choices and seemed to be getting worse rather than better. I was surprised she managed to get through, although there was only about 1% between her and Ruth.
Girlfriend and White Flag were much better choices and she seemed to be getting back to her old self again, and she obviously deserved to go to the final over Eoghan (who didn't?!), but by that point it was too late.
I loved every performance of Diana's, including her auditions, bar week eight. The only decent performance Emily's pulled out the bag all series is Creep.
Emily is nothing like Diana, so the only basis I can compare them is 'who do I like better?' and that's simply Diana - who beats out everyone in a popularity contest, for me.