Which Should I Use?
The answer will come down to this: do you have a competent graphics solution?
If you do not have a graphics solution by Nvidia or ATI, and instead are using an Intel chipset with Intel integrated graphics, then you can probably answer "no" to this question. Most of Intel's graphics solutions only barely meet the minimum requirements for using Windows Aero. This does not mean you can't use Aero, but it does mean you could run into some problems with performance. For example, my primary laptop uses an Intel 965 Express Chipset. Attempting to use Aero on this chipset does not result in a satisfactory experience.
However, if you have an Nvidia or ATI graphics solution, and you have the other minimum specifications met, then you will likely be able to run Windows Aero without any problems. Your graphics solution will be doing the heavy lifting, leaving the rest of your computer relatively free of strain.
With this in mind, it should be made clear that Aero's performance, while certainly poor on systems which do not have the hardware to make use of it, may not be the underlying problem if you are having problems with the performance of Vista. Remember; Aero consists of many 3D elements and makes use of many graphical effects. This will effect the performance of PCs that lack sufficient graphics hardware. If you have a decent graphics solution and your computer is still running slowly, then Aero is not your culprit.