Depends what's important to you. Restaurant Marrakesh wins the atmosphere war between the two. It is large, spacious, and impressive looking with lots of authentic looking Moroccan decor and gives you the feeling of being transported somewhere exotic plus it has the Belly Dancer and the show she puts on and little kids can go up and join her. Spice Road is small and quaint by contrast and can't compete - it is clearly the 2nd tier restaurant in it's pavilion just as La Hacienda de San Angel in the Mexico pavilion can't compete with the Santa Fe restaurant and it's volcano.
However, the food quality at Spice Road is where it clearly outshines Restaurant Marrakesh. The food is incredible. Food at RM is fine, I don't mind it, it's decent - it seems like they make their food with more sweet spices, like cinnamon while Spice Road uses more fiery and potent and savory spices, perhaps. Not sure of all the other differences in the cooking style but their menus are different.
I wouldn't say Restaurant Marrakesh is the worst restaurant in Epcot or World Showcase or anything, but it's definitely on that lower rung, to me, along with Tokyo Dining and Nine Dragons (probably the least memorable of all of the ones I've gone to). It's a bucket list type of thing - also, it's one of those where there is always availability, for sure, you can get a reservation like day of, easily, though that's also true about Spice Road, so one shouldn't infer anything about quality from that as there are many terrific eateries in Disney you can get a ressie for on the same day for one reason or another.
