With regard to being able to do those things, it’s not a binary or forced choice. You can do both with piper’s grip, and both with whatever Brian Finnegan does (obviously, since he does it). See e.g. https://www.youtube.com/watch?v=ODmqPvpxT50. Also note that uilleann pipers can also do intricate slides and half notes, and super fast ornaments, using the piper’s grip, i.e. https://www.youtube.com/watch?v=YMvBnEvuBuA for slow slides and half holes and https://www.youtube.com/watch?v=OAtEnKBvmbM or https://www.youtube.com/watch?v=onppWZ-yZHw for fast and incredibly intricate.
What’s more comfortable is the only answer, though.
What grip you use has nothing to do with doing slides or doing half notes or playing fast or playing ornaments.
Which is easy to observe when you see the same player using an end-joint-pad hold on a small whistle then instinctively switch to a middle-joint-pad hold on a big whistle. The player still plays the way they play.
It’s true that if I’m playing a tiny whistle (say, the smallest whistle in my roll, an Eb) and using my end-joint-pads on all six fingers the specific technique I use to get half-holed semitones is different than the one I use on Low Whistles when I’m using the middle-joint-pads on the index and middle fingers of both hands.
My first teacher/mentor, whose main instrument was Low Whistle and who used pipers’ grip, was of the opinion that he had more control over the half-holed semitones with pipers’ grip, and for sure he played those beautifully.
Here at 4:37 I’m half-holing the note that would be F natural on a D whistle, you can see how that’s done when using piper’s grip
Agreed, great whistle players who use the pipers grip execute slides and ornaments with precision—McGoldrck, for example.
It is about comfort and preference.