My experience over the years has mainly been with civies, my SO and the odd MPA or lap dance in a SC. Based on that, I think the answer is, it depends. I would say, it all comes down to the connection you have.
I can see married guys saying sex with a SP is better especially if they're seeking a particular act that a civie or their SO isn't into. Fulfilling a fantasy can be extremely hot and depending on what it is, may be difficult to find a civie who's into the same thing. So in that case SP sex wins. On the other hand, there's nothing hotter than meeting a random civie and within a couple hours you're tearing each other's clothes off and having sex like you both haven't been laid in years.
I'll never rule out seeing a SP but I'm lucky to have an open-minded bi wife who's up for just about anything. After all the years together, the sex is still awesome. We keep things interesting by going to places like Oasis, having threesomes and couples play. One of the best parts is not having to use condoms.
But that's just my 2¢.