The biggest difference is that a ported barrel directs the gases straight up. If you have to shoot from a retention position, i.e. the gun is basically alongside your chest pointed at your target, the gases/powder are directed up towards your face. Depending on the type of porting and/or round being fired, it could potentially cause some issues.

The gases/powder coming out of the cylinder gap of a revolver are directed more towards the sides. The top strap minimizes the amount of gases/powder going up towards your face.

I've never shot a ported gun before, so I can't say if there's any credence to the argument. Many say it's not an issue. Since I don't see porting giving a self defense gun any real advantage over a non-ported gun, I'd rather not deal with any potential issues and prefer non-ported guns for defensive use.

Just my opinion, FWIW.