I've not read a lot of Fraser Parker material, but what I have read is solid. I don't tend to like propless name guesses, because I find them to be too process intensive, personally, so I haven't really invested in his work on that area. Which means I have no personal opinion on Sync. I do have Ouija, and it's very interesting, and Sync seems to be along the same lines but a different approach.
I haven't read Turner's psych forces booklet, because I don't generally use psych forces (no need, really, in my style), but I have read several other of Turner's work and he's very good at what he does. Quite possibly he is in a league of his own when it comes to modern mentalism and we'll be parsing his works for decades to come. I would be willing to say that anything he puts out is probably worth the price tag, as long as you're willing to put the work into making it work for you.
Here's the thing about psychological forces - they're not magic spells. That's where a lot of people get caught up - they think they can just repeat what's in the script and it'll always produce the desired effect. That's just not true. First, you have to adjust the wording and the timing to suit your performance style and character. Then you have to work on that until you can do it naturally. At top skill, a psych force probably has up to a 90% success rate, if I were to guess. Meaning it will never be a bullet proof method. You're always taking a chance. A good performer can roll with the misses and turn them into a great effect anyway.
Something Peter Turner likes to point out in many of his products is that it's not about "forcing" one option - it's about invisibly reducing the other options and then guiding them to the final desired location.