In my opinion, MoTB and SoZ are equally as good, just in different ways. MoTB focuses more on roleplaying and getting your character involved in a deep story. It makes your character the center of attention, lets you get powerful equipment, and gives you cool, well thought out npc companions. However, if you like to explore, it sucks. Although you sometimes have a choice of order in which you want to accomplish things, there's otherwise little in the ways of free-roaming, exploration, etc.
SoZ, is about equally as good, but it focuses more on adventuring, exploring, and making a name for a party of people rather than one character. It puts less emphasis on the story and npcs, but they are still there, and have their interesting points. The dungeons are smaller, but they each have their own unique feel, which makes you feel like you're exploring more of a "world" rather than just an area. People say a variety of things bashing the game, but the main reason I think people don't care for SoZ is that it doesn't make you the center of attention. Sure, your party is important, but there are other things going on in the world to that you have to take the INITIATIVE to get involved in rather than having it spoonfed to you. Combat is also more difficult, and occasionally (espec. the last boss) I felt like I wanted to slam my keyboard repeatedly against the desk. But, it felt great once I finished these hard battles. In other words, SoZ includes a lot of lost elements in gaming that SHOULD make a return; it has put back in several things that others have taken out of games to "dumb them down." It includes exploration, difficult battles, a dynamic world and rewards the player for their ingenuity instead of only the time invested. IMO, it's slightly better than MoTB.