We should perhaps be careful to maintain a distinction between the size of the oil spill and its severity. Knowing that the Ixtoc I spill possibly involved a greater volume of oil doesn't necessarily tell us that the spill's effects were more serious. Perhaps somebody more knowledgeable than me can comment on the comparative repercussions of each spill.
(I can say from personal experience that everyone who grew up on the Texas coast during the 80s has memories of cleaning tar from his or her feet after every visit to the beach.)
I think it's probably too soon to say with certainty. But Ixtoc I was in relatively shallow water and was closer to shore. Deepwater Horizon was in (duh) deep water and much further from shore.
Ixtoc probably released more oil that ended up on coastlines as such; my understanding is that due to the depth, a lot of the products coming out of the Deepwater Horizon hole are getting "cracked" by the water pressure so there is less crude making it to the surface. Whether that is better or worse, I can't say -- it seems like probably better, but it could be worse for fishing and deepwater marine life, perhaps.
The thing that's most disturbing to me about Ixtoc I is how Pemex asserted sovereign immunity to avoid paying anything but the most minimal cleanup costs. BP may be a corrupt bunch of motherfuckers, but at least they don't have that to hide behind.
(I can say from personal experience that everyone who grew up on the Texas coast during the 80s has memories of cleaning tar from his or her feet after every visit to the beach.)