GPS "A" has a mediocre HDOP value, but an excellent VDOP. I'm going to assume its aggregate positional accuracy is some average of the two values.
GPS "B" has a better HDOP value, but lower VDOP. It's aggregate value is slightly lower than GPS "A".
Which device do you use? "B" will actually provide you with a better location on the ground, but it's overall accuracy is not quite as good.
And of course, all of these values change from moment to moment. On average, "A" may provide a better overall accuracy than "B". But how long do you wait for that average?
That said, since we're making a comparison between an external GPS and an internal GPS/GNSS sensor, You're going to go with the external all the time anyway. It's very likely to have better accuracy than the internal for no other reason than a bigger/better antenna. And why would you put it on there in the first place unless you expected better performance?