Electric is easier and cheaper to install, but far more expensive to subsequently run.
 
You will need to weigh up the relative initial outlay and running costs and decide whether you will be in the property long enough to benefit from it.

There is nothing 'eco friendly' about electric underfloor heating , unless you have access to renewable electricity generation , which doesn't include the UK national grid on a green tariff.
If you have a gas boiler already, and if the extension involves new/renewing the floor then wet UFH heated by the gas boiler doesn't need to be that expensive to install
and will cost much less to run
There are also thin wet UFH systems for existing floors ,but then there's a problem with insulation under this and how much could you raise your floor by?

Temporary used structures are a no-go for storage heating. Breakfast room and kitchen-if used only an hour per day or so- are not suitable for effizient usage of UFH.No matter what type of UFH.

definitely get a water based underfloor system. Trials in my line of business prove that in the short and long terms hot water systems are by far the more efficient. If the architect disagrees with that i would question his motives if his recommendations do not reflect mine. I have installed both these types of systems and have over 25 years of installation experience and can prove that the water based system is far more efficient. make sure that it is properley installed and use a supplier like uh-1 in doncaster to purchase your materials. you will be able to get them at trade cost
 
regards
 
nigel