All other things being equal, T-shaped Allen wrenches will work better than L-shaped Allen wrenches.
When I say T-shaped Allen wrenches, I'm talking about wrenches like these:
as well as the 3-way hex wrenches like the one you mention:
This is because the shape of these tools allows you to exert pressure from directly above the bolt rather than from the side. Exerting pressure from the side adds a lever element which can cause the tool to slip. Pressure from directly above the bolt keeps the tool held fast without that side leverage.
That's not to say that L-shaped wrenches don't have their place. They're typically much easier to get into tighter places. As others have mentioned, high quality L-shaped Allen wrenches will often have a rounded head at one end, which makes it easier to get at bolts from an angle.
It seems counter-intuitive, but these rounded heads actually hold a bolt very well. I can't recall ever having one slip (with the exceptions of already stripped bolts and/or user stupidity).
As far as Park Tools go, they are well respected in the bicycle industry as offering the highest quality tools available. That's not to say they're the only game in town. There are lots of companies that offer high quality Allen wrenches. If you don't trust Park Tools for whatever reason, buy your Allen wrenches from any high quality source that you do trust.