The benefit of downloading it as a plugin is that you can integrate it with your existing eclipse install. If you already have a lot of other plugins installed, that might be useful to you.
If you want Eclipse to be as fast as possible you should just download a very basic version of Eclipse and only install the parts of Aptana that you really need. Instead of everything. Aptana usually comes with too much bloat for my taste, I always disable quite a bit in the General -> Startup and Shutdown
section.
If you just need PDT than installing Aptana is probably your easiest way. If you also want to install cDT or something than installing a regular Eclipse package might be easier. If you want it fast, you'll have to disable some plugins either way. Aptana installs more than it needs by default.