Don't let some nay sawyers convince you, that you only need one holster. Including fanny packs, I currently have four holsters for my primary carry.
So, why not buy both.
I own a OWB holster for those times, when I need good concealment from a T-shirt or button down shirt un-tucked, but don’t' feel like warring an IWB because I'll be sitting at dinner, or a movie. And a good quality OWB holster that tightens down close to your body is very hard for anyone to detect.
For conveyance a quality paddle holster is very hard to beat. Like others have said, being able to remove the whole rig and keep the trigger covered for safety is a big plus. However, instead of a injected molded holster I would recommend a quality leather paddle holster. While it may not fit as tight as an quality OWB holster it is almost as tight fitting, and again very hard for anyone to notice.
When I worked for the Fed's I use
Pro-Fed Paddle Holster
a holster by DeSantis similar to this one, and traveled all over the country, including New York, Chicago, California, Boston just to name a few, with a G19 covered by a sport coat, and only once did a very savvy older gentleman who was guarding one of Florida's supreme court judges notice.
If I was to buy a paddle holster today I would look at the ones listed below.
FIST, INC.
Kramer Handgun Leather: Revolver holsters, concealment holsters, paddle holster, 1911 holster, ankle holster
Hope this helps