Not that knowledgeable about your specific bars, but silver, unlike gold, will oxidize over time, and maybe that is why one looks dull. If you own silver flatware and serving pieces, you probably have had to polish it when it "tarnishes"... i.e., oxidizes. Gold doesn't oxidize, that's why the gold from King Tutxs tomb was still so brilliant and shiny after being buried for 4,000 years.

If the bars are the same size, weigh them. They should be the same weight. If you know a good jeweler, he should be able to assay the bars to determine purity.

Finally, know who you're buying from... there are reputable metals dealers out there and and a few dishonest ones too.

Hope that helps, good luck.