Assuming that:

• There continue to be ridiculously cheap lives to be saved over the course of one's lifetime,
• Cryonics does not itself become ridiculously cheap when practiced on a larger scale,
• One values lives in a person-neutral way while in a mode of thinking that is not too far-biased to spur significant action,
• Cryonics does not increase your total budget for far purchases (e.g. reduced willpower-depletion from resisting luxuries),

The argument for contributing to (or promoting) efficient charity such as GiveWell instead of Cryonics makes sense to me.

However, I don't think any of the above are all that likely. Here are my counterarguments:

1. If GiveWell is given more attention and promoted more effectively, human lives will eventually become less ridiculously cheap to save. It's a simple matter of redirecting money that would otherwise go to less efficient charities. The requirement for this is more advertising/exposure. For the median smart person who spends significant time online and does not have a lot of money, time contributions towards advertising GiveWell and how ridiculously cheap it is to save a life that way may be more effective than equivalent cash contributions (given how relatively little-known it is).
2. If more people purchase cryonics, cryonics will become less expensive. The chance of cryonics working will also go up because it will be practiced in a higher grade clinical setting (which carries its own costs, but these come out of the normal cost of dying in the first world).
3. Part of the discomfort that cryonics causes (and one reason I think it is popular on lesswrong) is that it forces you to decompartmentalize between near and far. This is an important rationality skill. Developing it could easily trigger a higher tendency to take action on far issues like donating to GiveWell (or, generally, choosing an efficient charity like GiveWell over a more "fuzzy-optimized" charity).
4. Cryonics tends to feel like a very selfish choice. Thus money spent on cryonics feels like money spent on a luxury cruise, despite its potential positive externalities (such as making cryonics better/cheaper for everyone else). The part of the brain that insists that so many dollars must go towards selfish things is probably being sated by cryonics costs, leaving more room in the budget for charitable donations when all is said and done (assuming one does not develop the habit of rationalizing selfishness from the exercise).