1/2 stop difference isn't much in most situations, however depending on your camera when you're pushing the ISO limits 1/2 stop might matter. That being said, I have the Tamron 150-600mm and prefer to stop it down to f8 to get the best results. I'm happy to shoot at 12800 ISO with my camera though, plus Tamron's VC is excellent meaning I can drop the shutter speed right down assuming I'm not shooting fast moving subjects.
Basically what I'm saying (in a long winded way) is you need to know what ISOs you're happy with, and what you think your other exposure values may be to decide whether that 1/2 stop is the difference between a useable image and one that's not.