Hey there. Just got my Canon Rebel XS with 18-55 IS lens last weekend. I'm completely new to DSLR photography (still learning how ISO, Shutter, Aperture work together, etc) but I'm very interested in portrait photography (anything from kids to engagement portraits). What additional lens or accessory would give me the biggest bang for my buck to create better indoor and outdoor portraits (without breaking the bank)? A faster lens like the 50mm 1.8 (around $100) or an external flash like the 430EX (around $240), or something else I'm not thinking of? Thanks.