Results of my test: IATA 2 - TDOC 1
Well, I just tested them both out, with a real-life scenario. I'm an Australian passport holder, resident in Switzerland. I'm travelling to Saudi Arabia on business next week.
The IATA site asks for more details, so takes a bit longer. The TDOC site is simple and quick - but in this case gives the wrong answer, as it fails to take into account the fact that I am resident in a country other than that of my nationality. This may affect visa requirements for the destination country, and certainly affects visa requirements for the return trip.
Both sites correctly tell me I need a visa for Saudi Arabia (so 1 point each). TDOC incorrectly tells me I also need a visa to return to Switzerland if I intend to stay more than 3 months. IATA has already asked me if I have a residence permit, so knows I don't need a visa for re-entry (1 point).
So I think the TDOC site is handy and quick for those in straightforward circumstances - but then those in straightforward circumstances are less likely to need advice in the first place. If there's anything remotely complicated in the situation, I would go for the IATA site.