Nait 5i is the newer model with the RCA input and output jacks, which the older 5 lacks. The 5 requires the use of DIN 5 pin cables. The 5 can also accomodate the optional Flat Cap power supply, which the 5i cannot.
I owned the 5i for 6 months and absolutely loved it. If I had the money, I would buy it again. I did not have the chance to A/B the 5 and 5i.