In general Genesect is better than Heatran. Reasons:
Genesect has higher speed.
Genesect has less weaknesses( Genesect has 1 Heatran has 3).
More useful ability( who wants to hit Heatran with a fire type move?)
You can use both Physical and Special moves on Genesect.
In terms of being better, Genesect is an Event only Pokémon whereas I was able to catch my Heatran in Pokémon Pearl.
However before you consider my answer take your teams current type coverage into account first.