Here's my take.
The only way to approach the Crowley-Harris deck (and by extension the Liber T) for the first time is to pick an area to focus on and whittle away at it. Pick somehtig that interests you, intrigues you, perplexes you... so that when you start picking ast it you'll have a little extra oomph in your investigation. Give either deck a chance to seduce you.
So when you're deciding between them opt for the one that gets your motor going a bit. The Thoth is MUCH more detailed and scrupulous about the included details, as well as being the
source document. The Liber T is "sexier" if less accurate with regard to shapes and color, but it features those beautiful haunting decan images on the pips. The Thoth has the advantage of being much bigger and clearer than the Liber T, merely because of the size of the cards. So that may help you decide. In choosing, ask yourself what intrigues you... Astrology? Qabalah? Alchemy? Gender roles? Gnosticism? Mythology? Magick? Any of those topics (and more) are waiting in there for you... any one of them will lead you to the other topics. They are paths that cross and crisscross under the branches of the Tree and under the Stars.
Don't be scared by anything people have said about
Book of Thoth. It's not nearly as rough as people make out and often hilarious/heartbreaking; the trick is to just wade in and enjoy it. Pick up what you can and go back often as your curiosity dictates. Also, I always recommend it... and you should hold off a bit before you tackle it, but I love love love "The Wake World" in
Konx Om Pax, which you can find online (with helpful notes) in several places. It's a kind of fairytale journey through the cards of the Thoth written by Crowley, often literally using Thoth symbols and imagery to express some ravishing ideas about Magick and Will.
The Liber T is my deck of choice, but I'd read the Thoth for over 20 years before I picked it up. So I'd say pick your poison. What gets you all hot and bothered when you look at the two of them? Which deck will give you the best shot at exploring the topic? If it's colors or Qabalah, the Thoth is more accurate and scrupulous. If it's astrology, the Liber T covers a lot of stuff that Crowley didn't spend time on. If it's Projective Synthetic Geometry, a trimmed Thoth makes things much easier to trace, and the Liber T doesn't really replicate the effect. If you want streamlined images that aren't as adamantly Golden Dawn adherent in colors and forms, the Liber T will give you that.
Which do you reckon?