The running costs are determined by the insulation to a certain extent and assuming this has been installed correctly, I would run the heating for set periods and not constantly (unless you're cold ;-)) to save some costs.
Whether you use a water or electric underfloor heating the running costs will vary considerably 'without' insulation and not to bore you I'll tell you a (painful) short story.
I undertook an underfloor heating installation on the 'assumption' a floor had previously been insulated as promised by a Conservatory contractor.
After the first energy bill arrived my customer asked whether or not they were investing in the national grid with their floor heating installation, because of the cost.
At that stage I realised that NO insulation or very little had been used and much to my disappointment I offered to re lay everything out of my own pocket, tiles, heating, adhesives, labour...the lot.
I've never made the same mistake twice and know that the insulation is the KEY to effective underfloor heating. The warm up times are minimal, so timed, constant or any other combination shouldn't make too much difference when the system is effective.
