There have been quite a few threads on both of these central heating
controller products but I have never seen any direct comparisons.

Does anyone have an opinion on how they stack up against each other?

For me the Dataterm from WarmWorld is technically ahead of the Danfoss
controller (e.g. it looks back at last 5 days and uses fuzzy logic to
predict the future with a built in memory that learns the thermal
characteristics of your house/heating system. You set the temperature
you want at a particular time and Dataterm just does what it takes to
ensure that this is met winter, spring, summer, autumn).

Danfoss is a bigger company with everything that entails from that
(e.g. more customers, more marketing clout, better support?). But the
product does not seem as good, the Optimal Start will bring the
heating on up to 2 hours before the required time depending on ambient
temperature, but that's it. The chrono-proportional control cannot be
used if you have motorised valves.

Thanks