The Roland SC-55 was the must-have sound module of the nineties. Many game composers used the Roland SC-55 as a reference module to create their music (e.g. Doom and Descent). With it you could hear the music as the composers heard it - the way it was meant to be heard. Another reason for the popularity nowadays is the nostalgic feeling for those who actually owned a Roland SC-55 back in the day.
The Roland SC-D70 is one of many successors of the famous Roland SC-55 and is the last of the Sound Canvas family. Other well-known successors of the SC-55 are the Roland SC-88, SC-88Pro, SC-8820, and SC-8850. The Roland SC-D70 is actually a Roland SC-8820 with digital audio output ports and has full USB-functionallity to connect it to a PC, while the USB-connector for the SC-8820 was only used to transfer MIDI-data.
Every new device added new instruments (called patches) but also changed already existing instruments, which means there are big sound differences between the different modules, even though the games weren't optimized for it.I didn't get to enjoy a Roland SC-55 back in the days. I only heard of it years later. I listened to the music packs from MusicallyInspired (Brandon Blume), which were recorded using the SC-55. In order to listen to songs other than the packs he uploaded I decided to buy the module myself, which I did at the end of 2017 and I have to say that I really like it.
There are many songs that just sound great with the Roland SC-55 but there are also a few songs that, in my opinion, don't sound quite as impressive. I wanted an alternative for these songs. I could have used soundfonts, but they are often imbalanced between different instruments (e.g. quality, volume etc.). I was rather looking for something that reached the quality of the SC-55 and was also as well-balanced. So I came to the conclusion that I simply should get another Sound Canvas device. After some research I came across the Roland SC-D70, which I bought mid-2018. Not long after I was inspired to arrange these music packs so that others can also have an alternative to the Roland SC-55 Music Packs, which offer the same high quality.
Only you can answer this question. However, you shouldn't decide that based on the specifications of the devices but rather on which sounds you prefer. Compare both music packs and then pick the version that you like most. It's about personal preference after all.