Just putting it out there: Batman AC will release this fall, whereas Tomb Raider won't be coming out until Q2 2012. Should be enough time in that 6-8 months to scrape together the $100 and get both. (I plan to.)
More excited for TR, since I've been a fan of the series since the beginning, but Catwoman's appearance in Arkham City has me quite interested in that game too. I know it'll only be a brief cameo, but I liked those kinds of sections in Ninja Gaiden Sigma and Sigma 2, and it will keep me excited to keep playing the game to get to the next Catwoman section. I liked Arkham Asylum, but have to admit that I eventually got bored with the game and never finished it.