Huh. So if you're not using Heatran as a Stealth Rocker, you do have that free space for whatever you want. Between Roar and Toxic, it's a pretty tough choice. It kind of depends on whether you want to stop set up sweepers and rack up hazard damage or break down walls as well as bulkier sweepers. There isn't really a right or wrong here.
Also, just for fun, some other possible options are Flame Charge for making Heatran a pseudo sweeper, and Taunt for shutting down walls, hazard setters and set up sweepers.