ORTF a meter or so front of the performer should be a pretty safe bet. Without knowing very much about the situation I'll suggest that.
All things equal and with freedom of setup, I often dig omnis over cardioids in outdoor environments. Far less wind noise issues, usually more natrual sounding all around. However, the audience's behavoir becomes more of an issue if the atmosphere is more cockail party rather than everyone paying attention to the performer. To counter that somewhat you can place the omnis closer to the performer, which helps raise the performer's level compared to that of the audience, while not sacrificing the omni naturalness. I wouldn't space them too widely if setup close to a solo performer, 18" apart is probably fine.
Actually if the audience is cool and other distractions minimal, I'd probably use whichever mic pair is better. Personally, I tend to be more critical of the sound of low cost cardioids more so than low cost omnis. ORTF is likely the better way to go if you have good quality cardioids. If your omnis are better microphones, that could sway me that way.