Khalid,
The SS-USB gives two parallel ports worth of I/O's and SS-Etherenet give three PP's. On the other hand UC100 is a very compact size (just the size of a 25 pin connector cover) and gives just one PP. UC300 gives you 5 PP's worth of I/O's. UC300 also gives you two analog inputs and two analog outputs with these. I am using UC for their superior support for I/O's. A machine under development here in Karachi needs 46 I/O's so I went for the UC series. Price of UC100 is 80 Euros and for UC300 is 105 Euros. If you need one, do let me know. I am going to order a few in weeks to come.
Zafar