NV nudges you in a certain direction, then branches out, while Morrowind is truly open from the start. You can go to Balmora, Vivec, Hla Oad, you can explore the islands, etc. All of it makes sense from the POV of a new character, while fighting Cazadores in the first hour of NV is obviously a hardcore challenge.
NV, like classic Fallout, presents each area as a stage for a specific dilemma involving different factions, often functioning like an episode in a TV serial. Will you favor faction A or faction B? The player's actions decide the future of each location. In Morrowind you do things for people, but this is not necessarily reflected in some larger outcome specific to that area. The factions that exist are normally dispersed throughout the map and you do a lot more back and forth for that and other reasons, like catching the silt strider, visiting certain merchants or storing loot. You'll be visiting places like Balmora time and time again, even without a related quest.
Morrowind offers a main quest as an option, which you can skip entirely. NV's events unfold as you explore different environments. The latter's approach is more sophisticated, more grounded in conventional "game design" wisdom. You can play Morrowind for hundreds of hours with dozens of characters and never bother to "finish the game", that is, the main quest. Is this a problem? No, as long as you're absorbed in what you're doing for whatever reason.
Ultimately, I gravitate more towards Morrowind's approach. It's riskier and more ambitious, but it works in Morrowind due to the richness and depth of its world, which also can't be separated from its sandboxy mechanics like creating spells.
