The only difference is premeditation. Lack of premeditation is indicative of an inability to control ones thoughts and actions, which in my opinion is far worse.
really - perhaps consider another context - murder
someone plans to kill someone well in advance, with a cool head
vs
someone gets in a bar fight or catches someone breaking into thier own home, they overpower the attacker/burglar but in the heat of the moment, adrenaline pumping they go too far and the attacker/burglar ends up dead
most people would tend to take the opposite view to you - premeditated being much worse
you explain to your wife the drunken one off incident on the stag do, with lots of remorse, you slipped up, it won't happen again etc..
vs you explain to your wife that you actively signed up to a cheating website , contacted several women, eventually found one to meet up with - clearly not a mistake but something you planned, decided to do over a number of days, with a cool head
you'll find that though most women would be upset with either, most would be much more upset with the planned online encounter than with a drunken one off mistake
adrenaline, alcohol etc.. affects the judgement of every human being to some extent