Why is the choice not directly comparable? If it were possible to offer a blind person a choice between being able to see, or having a guide dog, would it be so difficult for the blind person to choose?

Still, if you can suggest better comparisons that make the same point, I'll be happy to use them.

These are good points and I'm suitably chastened for not being sufficiently thorough in checking Toby Ord's claims,
I'm pleased to see that GiveWell is again investigating treating blindness: http://blog.givewell.org/2017/05/11/update-on-our-views-on-cataract-surgery/. In this very recent post, they say: "We believe there is evidence that cataract surgeries substantially improve vision. Very roughly, we estimate that the cost-effectiveness of cataract surgery is ~$1,000 per severe visual impairment reversed.[1]"
The footnote reads: "This estimate is on the higher end of the range we calculated, because it assumes additional costs due to demand generation activities, or identifying patients who would not otherwise have known about surgery. We use this figure because we expect that GiveWell is more likely to recommend an organization that can demonstrate, through its demand generation activities, that it is causing additional surgeries to happen. The $1,000 figure also reflects our sense that cost-effectiveness in general tends to worsen (become more expensive) as we spend more time building our model of any intervention. Finally, it is a round figure that communicates our uncertainty about this estimate overall. But it's reasonable to say that until they complete this investigation, which will be years rather than months, it may be better to avoid using the example of preventing or curing blindness." So the options seem to be either not using the example of blindness at all, or using this rough figure of $1000, with suitable disclaimers. It still leads to 40 cases of severe visual impairment reversed v. 1 case of providing a blind person with a guide dog.

I don't understand the objection about it being "ableist" to say funding should go towards preventing people becoming blind rather than training guide dogs

If "ableism" is really supposed to be like racism or sexism, then we should not regard it as better to be able to see than to have the disability of not being able to see. But if people who cannot see are no worse off than people who can see, why should we even provide guide dogs for them? On the other hand, if -- more sensibly -- disability activists think that people who are unable to see are at a disadvantage and need our help, wouldn't they agree that it is better to prevent many people -- say, 400 -- experiencing this disadvantage than to help one person cope a little better with the disadvantage? Especially if the 400 are living in a developing country and have far less social support than the one person who lives in a developed country?

Can someone explain to me what is wrong with this argument? If not, I plan to keep using the example.

Regrettably, I misspoke in my TED talk when I referred to "curing" blindness from trachoma. I should have said "preventing." (I used to talk about curing blindness by performing cataract surgery, and that may be the cause of the slip.) But there is a source for the figure I cited, and it is not GiveWell. I give the details in The Most Good You Can Do", in an endnote on p. 194, but to save you all looking it up, here it is:

"I owe this comparison to Toby Ord, “The moral imperative towards cost-effectiveness,” http://www.givingwhatwecan.org/sites/givingwhatwecan.org/files/attachments/moral_imperative.pdf. Ord suggests a figure of $20 for preventing blindness; I have been more conservative. Ord explains his estimate of the cost of providing a guide dog as follows: “Guide Dogs of America estimate $19,000 for the training of the dog. When the cost of training the recipient to use the dog is included, the cost doubles to $38,000. Other guide dog providers give similar estimates, for example Seeing Eye estimates a total of $50,000 per person/dog partnership, while Guiding Eyes for the Blind estimates a total of $40,000.” His figure for the cost of preventing blindness by treating trachoma comes from Joseph Cook et al., “Loss of vision and hearing,” in Dean Jamison et al., eds., Disease Control Priorities in Developing Countries, 2d ed. (Oxford: Oxford University Press, 2006), 954. The figure Cook et al. give is $7.14 per surgery, with a 77 percent cure rate. I thank Brian Doolan of the Fred Hollows Foundation for discussion of his organization’s claim that it can restore sight for $25. GiveWell suggests a figure of $100 for surgeries that prevent one to thirty years of blindness and another one to thirty years of low vision but cautions that the sources of these figures are not clear enough to justify a high level of confidence."

Now, maybe there is some more recent research casting doubt on this figure, but note that the numbers I use allow that the figure may be $100 (typically, when I speak on this, I give a range, saying that for the cost of training one guide dog, we may be able to prevent somewhere between 400 - 1600 cases of blindness. Probably it isn't necessary even to do that. The point would be just as strong if it were 400, or even 40.