I have a Canon XTi and a Canon 10-22mm f/3.5-4.5 lens. I'm taking a trip to San Diego / Mexico, will probably do the Zoo, beaches, Sea World, etc. I was looking at: Sigma 17-70mm f/2.8-4.5 http://www.bhphotovideo.com/c/product/419582-REG/Sigma_669101_17_70mm_f_2_8_4_5_DC_Macro.html or Canon EF-S 17-85mm f/4-5.6 IS http://www.bhphotovideo.com/c/product/351548-GREY/Canon_9517A002_EF_S_17_85mm_f_4_5_6_IS.html I'm really unsure but I just want a little more versatility out of my camera. Looking for something under $600. Thanks for any help?