I think when we are frustrated with our current partner/situation the grass is greener syndrome can kick in. But I don't believe you should 'choose' the 2nd person. Attraction to others is normal for humans. It is what we do with those attractions that matter. I'd want to try and stick around to sort things out with my current partner if that fails I'd break it off. No one else really enters the equation for me when I am involved with someone.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!