the back to black version of TDS is far inferior to the definitive edition for multiple reasons, but one of the main ones is side separation. the DE fixed the overlap/cutoff issues between sides. not to mention vastly improved sound quality AND artwork quality.
both versions of the fragile are great, honestly. but the DE really does bring some extra depth and clarity to things that's not as noticeable on the original pressing. plus, with the low price point, it's worth getting. my only complaint is that the packaging is not as substantial as the original pressing, but that's a minor gripe.