The 950 is newer. The 945 was the first generation PDI that came out around 1991. The 950 is the second, came out around 1995 or 1996.
The basic sound between the two is going to be very similar. Same with the reverbs or whatever DSP stuff they add to the organs. Rodgers had an outboard reverb available for a while, which is what they integrated into the Trillium models around 2001.
The 950 is going to be more featured than the earlier model. But as a basic musical instrument they will be similar.
I remember that the 950 was a cheaper model than the 945. My guess is that they cheapened the construction. The 945 was rather expensive when it was introduced.
In any case, if you are struck on Rodgers, either one will do. I always recommend newer organs as support for them is usually more available. Believe me, long term support is not the big thing it used to be for manufacturers and distributors.