You know, I thought Spartacus was pretty silly at the beginning (mostly for the ridiculous over the top CGI gore), but then I kept watching and it reeled me in to the point of being a must watch show. I thought the "Gods of the Arena" miniseries was pretty good as well.
That said, I'm finding Game of Thrones to be much deeper and more complex than Spartacus. There are a lot more stories going on at the same time than there ever were in Spartacus. Not sure which show I would rate over the other at this point - I think I need to see the rest of TGOT first.
BTW, they are planning another season of Spartacus but unfortunately Andy Whitfield who played the lead role has Non-Hodgkin lymphoma.
I think I read somewhere that they will have another actor take his place which usually isn't good for any show.