Not sure about the Dimension, but I hear it is very accurate. I have had 3 TC Venture rifles and they are VERY accurate out-of-the box rifles. Just about anything made by TC is a quality product. Everything by TC is made right here in the USA too.
I built a custom rifle with a Shilen barrel on a Howa action and it shoots well under 1/2 MOA. Never did shoot it with the factory barrel on so can't comment there.
Howa and Vanguard are the same actions, both made in Japan. Howa makes Vanguard for Weatherby. I think they are very underrated actions. It's as if Howa took all the best features of the Remington 700 and the Winchester Model 70, melded them together perfectly, then added even better features than either one of those actions such as the M16 extractor, and angled integral recoil lug that breaks free from bedding like a dream, and a firing pin that removes from the bolt with only a 1/4 turn for cleaning. The Howa/Vanguard use 2 piece Rem 700 scope bases, feed flawlessly, and timing of the bolt and trigger is as easy as a 700.
I think you will be happy with either. The Dimension's switch-barrel capability in the field is very appealing and convenient, but the Howa will have a lot more aftermarket options for stocks, triggers, bottom metal magazines, etc.
I have a buddy who told me his mountain lion guide likes the TC Dimension because he can carry it in two pieces in his backpack so it's easier to follow his dogs. Once the cat is treed, he takes it out, puts it together, and lets his client shoot his/her trophy.
So you have to really evaluate what your needs will be because the two rifles you have in question have very different capabilities.
Hope this helps! Take care