You’re comparing two entirely different things here. DL will allow you to copy your discs 1:1 with zero video quality degradation from the original disc, while using SL will obviously require compession of the original disc (unless the original is SL, which is uncommon anymore), resulting in lesser quality - how much the quality will suffer will vary based on what progam you compress the disc with, how much compression is needed, what extras you retain or remove from the original disc, your own quality standards, etc.
I don’t believe Lightscribe exists for DL media YET, but I’m not sure, I don’t use DL or Lightscribe. I think that there are printable DL discs, but I’m not sure about that either. If you’re satisfied with the results you are getting from the compressed copies and Lightscribe is important to you, then keep on using what you are now. If you’re willing to sacrifice Lightscribe and higher costs in return for perfect 1:1 copies, get DL.
If you want to use SL Lightscribe discs and get better quality, there are ways to improve the quality of your compressed copies. DVD Shrink does a great job for a transcoder, but you are probably not getting the most out of it. Properly making the most out of it by using Deep Analysis and AEC settings will result in even better conversions. Beyond DVD Shrink you can use DVD Rebuilder, it’s also free and, if properly used, will always result in better conversions than ANY transcoder such as DVD Shrink or CloneDVD. It re-encodes instead of transcoding, you can use a free MPEG encoder such as HC Encoder with it. It’s a slower process, but the results can be great when getting the best conversions possible, especially with longer discs with 2 1/2+ hours of content.