Hi Robert,
I used the trial editions of both products for quite a while. The primary feature you gain in the professional edition of either product is IntelliSense. ( Tangible's free version does give some basic IntelliSense for a small subset of core assemblies, but I did not find it very useful in writing my own templates. ) I realize there's a few other differences between the 2 different pro-versions, but to me, they aren't that important. Have a quick look at them, and if they don't standout to you either, then it's probably not that critical.
As the key difference is IntelliSense, I would still suggest that you start with a quick trial of both versions. They have surprisingly different "feels" about them, so you will likely gravitate toward one or the other. (If it helps, think ReSharper vs CodeRush)
Personally, I found Tangible's to have slightly more features along with more samples, etc. ( Tangible also has things like UML diagrams, etc, but I didn't care much about that. ) However, in the end, I found Clarius's Editor to be more natural and comfortable for me (quite possibly because I used it first, several months before I found Tangible's)
Once you find you preference, then decide if you want IntelliSense and pay for the upgrade. ( It's worth it if you find yourself doing much with T4 ).
As a small side note, the advanced debugging features of Clarius in the pro-edition have been pretty useful on a couple occasions, too. But overall, the "feel" of one product over the other is a much more important factor in making a decision.