50:50 indicates the founders want to retain some reasonable chance of staying friends with a good working relationship.
51:49 indicates that one or other needs to be in control and it will end messily.
Distributing equity is as much about feeling fair as being fair, so fighting for a minor difference is really not worth the effort when (not if) something goes wrong.
For the canonical post on this subject, see Joel's answer to a question on allocating ownership fairly.
You revised to ask for personal experience, which I have of exactly this. I'm not going to pour out the full messy story, but in short it is far too easy for the majority holder to pull rank in the inevitable heated row about (trivial technical detail that doesn't matter at all / marketing strategy / pricing / insert trivial topic for heated disagreement here). The next day there will be apologies all round, but that emphasis of imbalance spoils the vibe from the first moment it's triggered. There were other things in that tale, of course, but much stemmed from the majority/minority and its' use.
"I must be CEO of course" "I'll have 51, you'll get 49" are now personal red flags 30 feet high, illuminated by floodlights.
As an alternative, particularly if you are of a similar background or skillset I much prefer a three way split - which could be 33/33/33 or 40/40/20 as suits, and depending on the level of involvement. Someone more mature who can knock the children's heads together from time to time. Perhaps someone with a more business or sales background rather than yet another techie.
I can't emphasise this enough, but there is something about the dynamic of three as opposed to two that leads to much more pleasing ways of playing with others - and less taking the ball to run home.
Lastly, if you're tempted to say "it wont happen" It will that's guaranteed. You'll disagree on a lot of stuff - how you handle it, and how you're set up to handle it could make all the difference!