Do you like Ford or are you a Chevy man? It's sort of that choice. Some like the Bemis, some like the Toto. The Toto seat will match the color closer, but since the seat is not porcelain, it will always be slightly different. Also, the Toto will match the rim shape probably better than a generic (the Toto will be pretty much even with the porcelain with no overlap). The Bemis comes off easily so you can to a thorough cleaning, the Toto does not. As to longevity, I can't say. Some love their respective ones. Most of the Toto seats have a moderately deep 'cup' shape to place your butt where it needs to be. I don't know the shape of the Bemis which may be flatter. Some people like to perch near the front, some like to sit back. The Toto will only be comfortable if you sit back (and they do that so your waste product hits the water spot more readily). If you liked the Bemis, you can probably expect similar asthetics with a new one. Is there another dealer in town that stocks these? You may be able to check one out to determine the differences. It's hard to do with a picture or words...it's best if you can actually sit on one.
The Toto seat has a curved top cover, and really isn't well designed to sit on, although it won't hurt it. It's designed and meant to be a cover, not a seat. Not sure how the Bemis stacks up on that front. They aren't hugely expensive, but you probably don't want to buy one, then hate it, and have to buy another. Good luck. I'm sure you'll get some other comments.