Have both. When I use a grid, I use the 30 most of the time. I'm usually fairly close to the subject with a dish to begin with and there's quite a bit of loss of light intensity with the 15.
I only own a 30 and use it all of the time .. a 15 would be pretty tight, almost a large spotlight.
Even with the 30 you can't adequately cover much more than a half length shot at optimal dish placement distance of about 3 feet. Once you move the dish away from the subject you will get wider coverage of course, but you also start to lose the unique BD lighting quality.
"Portraits" is a way-big field. I use a beauty light primarily for fashion/beauty-style headshots. Whether I use any grid or not depends on how tightly I need to control the lights area of coverage. I'm usually on location, so that's where it makes a difference: How tightly do I need to control the spread of light?
If I had a run of jobs where the answer was coincidently "really tight" for every location, then I'd wind up using a tight grid a lot that month. The next month, maybe not.
