Naxxramas vs Ulduar

[SirVAO]

Does anyone have experience how different will be the progression through Ulduar as opposed to Naxxramas, considering only 25 difficulty?

Assuming of course that your raid group entered Naxx with 3.6k GS, with little Naxx experience, and shall enter Ulduar with 4.2k GS, with little Ulduar experience.

Which instance is longer and which one is more difficult (by difficult I mean easier to screw up)?

[Crystalleia]

Iirc, Ulduar is mechanically harder than even ICC (LK HC is comparable but that's about it), so there's your answer.

It's also what makes it fun, I suppose.

As for length, Naxx has 15 bosses, Ulduar 13 (14 with Algalon), but the actual time spent probably depends more on how much you're wiping (which is probably more in Ulduar).

Also, Ulduar has Hard Mode! so that should be another point of difference as well.