i vote for the one in better condition as stated by another member you want to give yourself the best shot at it specially since it's your first aro. how big is the one you're looking at? i currently have 3 aros, i bought them when they were about 6 inches, i'd stay away from the 3-4 inch ones for me it's more of a challenge to rear them when they're that small. aros are pretty picky eaters, i've heard a lot of people say that one of the top killers of young arowana is poor nutrition, i find that with the bigger ones it's less of a problem.
also like another member said ask the lfs show you a feeding before purchasing, if its a good fish they should have nothing to hide, this accomplishes a few things for you 1. if the fish has good appetite it's probably healthy 2. you can find out what the lfs has been feeding it so you know what kind of food to get when you bring your fish home 3. your aro will be full by when you get home (the fish will probably not eat for 2-3 days after you bring it home as it adjusts.)
that's probably more than you wanted to hear from a noob though....