I am not familiar with the functions of the original Slingbox. So my choice when purchasing was the Slingbox Solo or the Slingbox PRO-HD. Since the PRO-HD allows up to 4 video devices to be connected simultaneously, while the Solo allows on one, I decided to go the PRO-HD route, which now allows me to watch my programs saved on, as well as streamed live through, my Tivo in SD video resolution mode, AND allows me to watch those HD TV channels that my cable signal provides in Clear QAM format, through the Slingbox streaming in HD video resolution. It's nice to have those two input choices, and perhaps I will add a third (and maybe a fourth?) at some time in the future should I get another media device installed with its signals going into the Slingbox.
That being said, if you TV service is all standard definition, or you are happy with viewing in just standard definition mode, then there may not be any particular reason to upgrade to REPLACE the original Slingbox -- but if you wanted to add a second Slingbox to your service, then I would probably select the PRO-HD for maximum future flexibility in connections.