Makes no real difference, but AC was generally used as it's obviously simpler and cheaper - 'occasionally' DC would be used, so as to reduce any hum from the heater wiring.
One obvious use for DC heaters was the antique AC/DC radios etc. where if the mains was DC the heaters obviously ran off DC, but if the mains was AC then they got AC power instead. Funnily enough I've got an old HMV radio to look at currently - which uses U series valves, with 100mA series heaters.