I guess it'll all be down to marketing. Wizz look professional judging by their website's contents.
Let's hope they win!
Air Polonia only operate Stansted-Katowice 3 times a week, Wizz operate Luton-Katowice daily.
But Air Polonia operate from Stansted to other Polish cities (not seven days a week) including Warsaw.
Wizz's website is better and quicker.
Air Polonia will charge £106.38 for a round trip on 7-11 June, Wizz will charge £48.30.
Air Polonia will charge £106.38 for a round trip on 14-19 July (no flight on 12th July, my original choice), Wizz will charge £80.30.
Wizz flights are 0730/0805 in/out of LTN, APN inbound is 0845, outbound 1455.
Wizz operate newish A320s, APN 734s.
None of the above is conclusive, but I'm with Wizz.