That choice honestly depends on you.
One of your first considerations should be the speed of your lens. If you want tight focus on your subject with a blurred background, go with a faster lens (like my recommendation below). If you want good focus from the subject to the far reaches, get a much slower lens.
I'd personally go with a prime. The best I'd recommend is a 50mm f1.4 lens. It's a lot more expensive than the 1.3 (which runs about $100) but it eliminates some of the major problems of the cheaper lens. Then again, I'm not sure if it would work with the Rebel, but the $100 lens would be a fine choice. Just be careful around a lot of parallel lines.
There's also an 85mm (I forget the aperture off hand) that will get you a bit of a tighter shot, but that's up to you as well.
Not having a zoom can be both a detriment and a benefit -- especially considering you'll still have your Rebel's lens. With a prime, there are fewer lenses for the light to pas through, which leads to sharper images.
Your legs become the zoom. The benefit of this is that it takes much less time to get your shot. Your babySLR* becomes almost a point-and-shoot once you're adjusted for the light conditions. The detriment is that you will need familiarity with the lens before hand, so you can predict the shot before you take it. It's a lot slower to keep backing up, hoping to get that candid shot and with your camera up, waiting to frame the scene, you're more likely to run into something behind you. Knowing your lens will help you know ahead of time how far back to be to get what you want.
tl;dr. lick
ling to view the boring shit.
