You might be able to create your own chart with paper and pen. Across the top write the models you are interested in. Such as ST1501 for the regular 15" string trimmer, another model, etc. making 3 or 4 columns depending on how many you want to compare. Down the left side of the paper write down which attributes interest you. Size of string, does it come with a battery or not, which battery, etc... Then it makes it easy to just jot down the attributes or put yes/no for each. Oh, haven't seen an online comparison chart. That would be a nice thing to see.
Greg, you want the 15" string trimmer, either straight or hinged shaft (no real difference). The only reason you might want something else would be if you need an edger, then waiting for the power head system might suit you.