All things being equal regarding condition/pads/adjustment condition etc. there really isn't a great deal of difference in tone/mechanism/performance. Most is of a cosmetic nature save the rolled tone holes. Now I own 10M/6M horns from the 30's through to my 1967 Silverplate 10M Tenor, and I can tell you that the later horns play and sound wonderful, with sometimes better intonation than there earlier brethren. That being said, they aren't as praised by collectors as they fall outside the more coveted (read romanticized) "golden" era of ladyface horns. If you go for the 1955, and again if all is in good nic there is no reason why you shouldn't, make sure you get it for a good price. You didn't mention the finish/condition on either, or the type of horn (alto/tenor/baritone) so I can't begin to offer a valuation. Best of luck.