Done both,
It can be fabulously kinky and electric having a friendship with a friend who is fucking your wife, but comes with risks,
An emotional attachment is easily formed between them and its easier for them to have extras without you realising it,
If she becomes pregnant he will know and thats a added complication if youd prefer him to be an absent sperm donor
Then what happens when you or her say enough, that can end the friendship between you all
It poses more chance of other friends, family finding out,
They can become possessive, refuse to accept the end especially from you and turn nasty or emotional,
On the plus said the last friend that fucked my wife is now my best friend, we/they have remained friends too and have an electric friendship even his wife has commented on his closeness between him and my wife, even if the sexual element is long gone the bond between them hasnt,
With a stranger its easier to have more control of his access to your wife,
Less complicated,
Harder to find a decent man,
It is so exciting seeing her in the hands of a stranger and submitting to him,
You do not have the same sort of electric relationship with him as you have with a friend being her lover.
Easiler for you both to end and move on.
Better security and less chance of friends finding out.
The list goes on.....