i am going on a 3 month travelling trip and going to take some pictures. i am debating whether i should get a new lens for the trip. the pictures taken by the old lens that came with the camera sometimes look sharp, sometimes don't. but i don't know if i should get the 28-105mm f/3.5-4.5 or 28-135mm f/3.5-5.6 because i am not sure if the quality difference is big enough. should i stick with the stock lens for now and save more money so i can trade up later to something better than the 28-105mm? or is the 28-105mm a worthwhile investment? i prefer zooms for now, for the convenience during the trip. thanks!