If price really didn't matter, I'd look at Canon lenses. I'm not sure you'd get all that much better lense unless you went upper end, and if your anything at all like me, your probally not good enough to tell the difference by looking at a photo. That's why we don't have high end digitals.

If you have a camera store nearby that stocks both, bring your camera body in a try them. Shoot off tripod, so camera motion has no effect - camera stores also have tripods, so no need to bring one.

They may well be the same lens, but one in a Sigma mount and the other in a Tamron mount. Unless you are a Japanese detective with considerable skills or very much an industry insider, it is impossible to know who actually made the equipment.

Companies like Cosina and Sigma make equipment for many of the big brand name marketers like Nikon and Canon. I have a telephoto lens marketed by Vivitar that was actually designed and built by Perkin Elmer, who were the prime contractor on the Hubble Space Telescope. Minolta manufactured the Leica CL and Panasonic makes some of the Leica digitals. Nikon currently has two film cameras on the market, and one is made by Cosina. It was also sold as an Olympus, Rollei and Yashica. It may have been sold by Canon as well - not sure. Cosina also make the Voigtlander camera system, Epson digital and some lenses for Carl Zeiss in Leica and Nikon F mounts. Kyocera built the Contax under license from Zeiss with Zeiss lenses, some of which may have been made by Kyocera. Kyocera also made the Yashica brand cameras - or at least marketed them.

Tokina, Tamron and Sigma lenses are generally very fine from an optical point of view, though it can vary from lens to lens. With computer controlled lens grinding, it is pretty difficult to find a really poor lens now unless you buy a complete no-name brand. They have a reputation for using cheaper lens mounts than companies like Nikon or Canon. They also have the advantage of selling in a wide range of mounts, so can produce lenses in great quantity, making the lenses a bit cheaper than those of the major marketing firms.