I have the 103D and owned a S790 and now the newer s6200. There's no difference in picture quality and upscaling between the s790 and s6200 that I can see. If you turn off all image enhancements on both the 103D and the s6200, BDs look the same.
DVDs look different between the 103D and the 6200. I payed $599 for the 103D so DVDs must always look better !!
Seriously though, only you can really decide what you like. Some people don't like Darby or any processing .... If so, don't waste you hard earned.
I have a Darblet in two of my setups. IS that the only thing the 103D adds?