What's the difference? I've only heard of the CX25S. Maybe the 50SS is just a different name for the same piece of equipment and could depend on where you are in the world.
If you do end up getting a Bellman steamer I highly recommend drilling out the steam wand tip. I'm assuming it's got the same tip as the CX25. When I had one I made this small modification and the result was greatly improved steaming performance. As it comes the tip has a very small hole, about 0.8mm. This causes it to be very slow to heat the milk, but you end up adding lots of froth. I drilled it out to 1mm and it was much better after this - faster to heat and easier to control to get good micro foam.