I don't necessarily agree that one plays 'better' than the other, but Spartacus has more of an immediate payoff, IMO. If you're looking for weekly thrills, Spartacus delivers. The show is quick, bloody and coarse. As well it should be. The dramatic situations of Spartacus are very plebian/Shakesperean in terms of theme and delivery. There ARE politics, to be sure, but Spartacus is more concerned about the bloody consequences of dealings gone foul. Everything in Spartacus is all build up for the inevitiable collapse of the characters' schemes. Rather appropriately, Spartacus is completely about the payoff.
The politics of of Game of Thrones feel more 'real world,' and interwoven. If continuing layers and interweaving intrigue is not your thing, you're likely to consider the show a little slow. Thrones is not about the payoff. It's about the whole process. Payoffs are just another event, leading to the next series of events. If you aren't into watching the evolution of the thing, and only want to see how it all turns out, get ready for a long and frustrating ride. Real world politics don't just 'end' and the show is not about fighting and blood. Fight and blood are just byproducts when others see through the gilded smiles and fake promises as they converse.
Just the scene from last night where Varys and Petyr are talking about the throne just before Joffrey walks in on them is deep and layered, and if you take into account all the different ways in which the characters have been influencing the world around them, it takes on so many meanings and so much weight. But in order for that to matter to the audience, they have to be interested in the political drama. Ned finding out the truth of John Arryn's death is not the endgame of the storyline. It just sets up the next thing. And it goes on and on and the complications twist and writhe and change continually.
The MOST interesting about Thrones, to me, is watching these layered, complex and wholly fascinating characters react to those situaitons and seeing the cycle of intrigue continue.
Another difference between the two is their source of origin. Spartacus isn't nearly as complex as Thrones, and it was written as a one season TV show with the potential for a second if people liked it enough. If you know anything about Spartacus, the popular versions or what little is known of history, all the foundations are set up for any iteration. If they didn't continue it, it serves as a suitable 'prologue' to the myth of Spartacus. Because of the tight framework and the knowledge the producers had of how many episodes they had, they were able to plot just enough into each episode to move things along to the payoff.
Thrones isn't written like that, wasn't conceived like that, and there is considerable shoehorning that has to be done to fit that book into ten episodes. I'm frankly amazed at how well they were able to pull it off. But wasn't conceived as a ten episode TV series. Martin wrote it from the standpoint that it would be something unfilmable, so he put everything in there that he wanted to.
But its origins as a novel are evident from the very first. They made it as all-inclusive as they could, but complex books about dirty politics just aren't as accessible to general audiences as a TV show crafted solely to appeal to general audiences. Spartacus has a beautiful simplicity to its storytelling, a sort of plain genius that just fucking works. It carried on into the prequel, Gods of the Arena, and you can tell that the producers have this thing figured out.
It helps Spartacus that they can put in action, sex and intrigue in a single episode by centering it on a very small group of people who exist to fight and those who directly control their lives. Let's see how things change once Spartacus and his rebels go to war. I hope Starz doesn't skimp. I don't need epic wars in every episode. I don't need Helm's Deep every week, but there will be some very vicious fighting going on as Spartacus' revolt grows to thousands. I can't wait to see how the showdown between him and Glaber plays out on the screen.