Looking quickly at this info I see GCT is a shorter trip, less ports, older ship (2000 vs Globus 2007). Is anyone a WWII history buff? Globus include Nurenberg, this is a big draw for some folks.
GCT prides itself on being a good tour provider with lots of different information about your destinations, for instance, you can visit a local school in Prague and have lunch there.
I also noted Globus seems to include more tours where GCT a lot of the tours are "optional" meaning you pay extra. For instance The Melk Benedictine Abbey.
Another difference is GTC starts on Prague and Globus starts in Budapest.
I did a river cruise in 2000 that started in Passau and ended in Budapest. It was 6 days and that was long enough for me. Eastern Europe has come a long way in these 7 years so I'm sure things will be better for you.
I am going to email you my email address for further info.