In reply to wp1801:
I'm not sure there's such a thing as 'long distance hiking boot' - just boots of various weights and stiffnesses that will make them more or less suitable for a particular long distance walk at a particular time of year.
As is already clear from this thread, the debate re shoes versus boots has arguments in favour on either side. Fundamentally it's a personal choice based on what works for you in a given situation. There's no one answer, you have to expect some trial and error in pursuit of your own opinion.
As others have said (but it bears repetition) - more important than anything else is footwear that fits you comfortably. If you're coming at this from scratch then you may have to try more or less every shoe/boot in the shops on before deciding on a particular model. And whatever you do, don't set out on a big walk in a boot/shoe that you haven't already got some decent mileage in!
For what it's worth (no more than anyone else's), my own preference is for trail shoes on predominantly hard, well drained surfaces and in dry above-zero weather. However if it's cold, wet, boggy, if there's a lot of rough off-trail ground, or if there's a fair bit of snow on the ground, then I would be inclined to opt for boots. Carrying heavy packs puts a lot of strain on feet/ankles, so if you're new to that then something more supportive may be good at least initially.
The precise distance would be secondary for me to all of the above considerations, though all else being equal I do find shoes more comfy for multi-day walking.
On the WHW it'd be shoes every time for me, even in winter. It's mostly gravel bashing on very easy surfaces, and I'd always prefer the lightness and cushioning of a sturdy trail running shoe for that sort of thing. I'd just accept wet feet as the tradeoff and bring enough socks.
On something much tougher and wetter like the Cape Wrath Trail - especially in spring with snowmelt and the possibly a lot of snow patches to cross, and the likelihood I'd climb mountains along the way - then the choice would be a lot harder for me and I can see myself going for boots.
For the TMB I can't say. I guess it'd depend how early in summer I was doing it, and thus how much snow I could reasonably expect to be walking on over the high passes. If you are kicking steps or need crampons then as a rule you've exceeded the remit of a shoe (there's always someone who'll disagree of course).
In terms of choosing trail shoes, this group test from last year could be a good place to start: https://www.ukhillwalking.com/gear/reviews/mountain_footwear/trail_running_...
And while I'm recommending material from the archives, there's this article on how to walk the TMB which could be useful: https://www.ukhillwalking.com/articles/destinations/how_to_walk_the_tour_du...