New Lens or Flash? -- Stage, low-light photography.
I'm relatively new to dslr photography. I've got a Canon Rebel XSI camera, and a 50mm f/1.8 lens, which I've been using to take some photos at theatre shows. I'd like to start tinkering around with taking promotional photos for some performer friends of mine, but am not sure where to put my resources... a decent off-camera / slave flash, or a new lens? My budget is around $500.
I've been looking at the Canon 85mm f/1.8, as it looks like a nice lens for fast, low-light situations. Looks like you can buy it used for ballpark $350 which is pretty attractive.
I've also been looking at the Canon EF-S 17-85mm f/4-5.6 lens, as it seems a bit more versatile, and could be useful for taking photos outdoors. Many seem to recommend this as a good outdoor lens for walkaround photography, and would be useful in settings that are brighter than a show on stage.
I've been poking around a bit on Strobist.com and trying to learn more about flashes, but I still feel pretty hopelessly lost when it comes to this stuff. Would it be in my best interest in shelling out for a new Canon Speedlite TTL flash?
What do you guys think? Any and all comments and suggestions weighing the pros and cons here are welcome and appreciated!
an example of a pretty ok photo I took of a friend of mine on stage a few months ago. Here's