For vex, using an angle grinder is usually going to be over kill unless you are cutting most everything lengthwise. For what you are going to use it for (cutting c-channels into half wide) an angle grinder would be quicker, however a dremel tool will work nearly or just as well since they wont be a single continuous cut, but rather small ones connecting the center holes. If you were instead cutting plates lengthwise into single wide pieces, an angle grinder could be better.
You also want to look at what materials you will be cutting. Steel is going to take much more time than aluminum to cut properly and will wear more on the cutting disc. If you are only planning on cutting vex pieces, usually aluminum, a dremel tool will be more than sufficient. If you are planning on cutting a lot of steel and using it for larger (metal cutting) projects outside of vex, an angle grinder will be better.
This brings up the next point, versability, a dremel rotary tool is going to be much more versatile than an angle grinder, with a lot more bits you can use. The cut-off wheels will also be thinner and with a lower diameter than an angle grinder, allowing for much more fine detail and precision. The dremel is also going to be easier to use and quicker to learn than angle grinding.
You also have to think about the amount of you are going to want to put towards replacing bits and cut-off wheels, as well as the initial cost of the tool, if you go to your local hardware store and compare prices and types available that will give you a better idea of what you want.
I personally use a dremel tool for all my vex cutting since it's so versatile and allows for much more precision on the smaller scale. For metal work outside of vex, I almost always use an angle grinder.
Regardless of what you buy, always wear a mask to filter out the aluminum and steel dust (steel will mainly spark and keep in larger particulates, while aluminum will turn to powder) and wear safety glasses at all times.