I've used TGM, but not touring plans so I can't give a direct comparison. I really liked TGM, but wouldn't bother a 2nd time. Why? Because I got so much info the 1st time, in that TGM gives really good explanations of WHY things are the way they are. Now that I know why, I feel I can make pretty good estimates using the same thinking. That is, it's not the numbers that are important but the explanation behind those numbers to understand how the disney crowds 'work.'
It's the same way I plan how to tour a park. A detailed plan is not as important to me as understanding why certain things are busy at certain times. Once I understand that, I can make up a plan on the fly.
Specific case: TGM was wrong on only 1 day out of 10. It was MK. When we went, we thought it might be busier than suggested and we watched the signs around us. We could tell it would get much, much busier the whole day. So we dodged that zig with a zag to AK and found it wasn't busy at all.
If you've never used either TGM or touring plans I think it's definitely a great idea to try one. After you try, you can judge for yourself. $20 isn't much when we had 8 people visiting for 10 days. That's $0.25pp/day.
However if you have touring plans I have no idea if TGM is any better. Maybe they're the same. If they're the same it seems like a waste to buy 2 of the same.