Earth Power, dammit. Why does everyone forget about Earth Power? D:
Modest LO Hydreigon vs. 252/252+ Heatran: 97.4% - 115%. He doesn't need EQ >:(
Oh and EQ doesn't OHKO Specially Defensive Ttar anyway, so even less of a reason. I do admit Ground/Fighting is bad coverage, but you don't need attack EVs when you have U-turn to get you the hell out of the way, since Ttar carries superpower. Sure you could invest in attack for U-turn only, but then you lose your power advantage over Latios, which sucks.
Honestly though, the choice between Hydreigon and Latios depends on your team. If you laugh at fighting types in general, then Hydreigon will only keep you from having a pursuit weakness, will lure said fighting types, and pack a bit more of a punch than Latios (modest vs. timid). On the other hand, if you think Tyranitar is nothing more than plushy set-up fodder, then you have no reason to not run the fastest dragon in the game.
Of course team building gets more complex than that but it really only boils down to how much you hate fighting types versus how much you hate Ttar. if both take you out though, well, you may need to reconsider your team.