It's worth noting that you should be considering what Pokemon actually use Hidden Power and what might be switching into them, since those are the only times Ditto will use Hidden Power. So you'll have things like Alakazam, Celebi, Espeon, possibly Gengar (but rarely), Heatran, Jolteon, Lati@s, Magnezone, and Venusaur...those are most of the big ones. Few of those are the sort of threats you should be using Ditto against; it's a poor answer to Celebi, Jolteon, and Heatran without Earth Power, though it can be useful against the relatively rare Nasty Plot Celebi. That said, I think Hidden Power Fire would probably be the best choice. It's much more threatening for hitting Venusaur, and if Lati@s is carrying Hidden Power, it's probably not on a rain team, so HP Fire would be a good choice to hit Scizor and Ferrothorn switch-ins. Alakazam also likes being able to hit Scizor and Ferrothorn. You'll need other answers to Dragonite (and other dragons) anyway, since you really don't want to rely on Ditto copying an opponent's Hidden Power to check dragons with HP Ice.
So if it were up to me, I'd go with HP Fire. HP Ice is better in general, but given Ditto's specific function, I think it would get more utility out of HP Fire, since it would likely be able to KO one of the checks your opponent has to one of their own special sweepers.
EDIT: I forgot about Thundurus-T; if it's a big problem for your team, you'll probably want to go with HP Ice for the ability to actually threaten Thundurus-T itself.