The funny thing is, (I don't have any inside info here, this is all pure speculation) I wouldn't be shocked if the AMF position ended up being more competitive of the two, despite the lower salary.
Normally non-profit salaries tend to be lower than for-profit salaries even when skills are equivalent because people are altruistic and therefore willing to work for less if the work is prosocial and meaningful and fits their interests. For example, the position of a professor is more competitive and has a higher skill requirement than industrial R&D jobs in the same field, but the latter is more compensated.
I believe that in the EA community this effect is much more pronounced. Some (not all) EA orgs are likely in a somewhat unique position where even if you offer 28-35k and less, you may still be getting applications from people with degrees from top schools who could be making double or triple or quadruple that on the open market, and at some point when you notice that your top applicants are mostly motivated by impact and not money you might become uncertain as to whether or not offering more money actually further improves the applicant pool.
In such an environment, salaries aren't exactly set by market forces in the same way as a normal job. Instead, they are set by organizational culture and decision making. This is likely all the more true for remote roles, where lack of geography constraints makes expected pay among equally skilled candidates even more variable.
Some people see the situation and think "so let's spend less money and be more cost effective, leaving more money for the beneficiaries and the impact and attracting more donations from our reduced overhead". They aren't in it for money, and they figure all the truly committed candidates who make the best hires aren't either.
Other people see the situation and and think "nevertheless, let's pay people competitive rates, even if we could get away with less" either out of a sense of fair play (e.g. let's not underpay people doing such important work just because there are some people who are altruistic enough to accept that, that's not nice) or the golden rule (e.g. they themselves want a competitive salary and would feel weird offering an uncompetitive one) or or because they figure they will get better candidates or more long term career sustainability and personnel retention that way.
One of these perspectives is probably the correct one for a given use-case, but I'm not sure which one and reasonable people seem to diverge.
(Of course it's not just personal preference - some organizations, and some positions inside organizations, have more star-power than others and so have this option more. And it's also a cause area thing - some cause areas perceive themselves as more funding-bottlenecked where every $2 in salary is one less mosquito net, while others aren't implicitly making that comparison as much, pouring extra money into their project wouldn't necessarily improve their project and the true bottleneck is something else.
Even if one believes they can make more impact at AMF, they would have to give up 20k pounds in salary to pass on the content specialist role. We learned recently to consider earning less, but this may still be quite the conundrum. What do you think?
As far as the personal conundrum goes, I guess you have to ask yourself how much you value earning more, and consider if you'd be willing to pay the difference to buy the greater impact that you'd achieve by taking the position you believe is higher impact to take.
(Looking at AMF, it appears the bulk of their staff have "operations" in their job title or appear to be operations-oriented.)