If you're thinking about where to get the most bang for your buck, that would be much more effective than providing a subsidy. The idea behind it is that with the infrastructure, you're enabling people to do something they can't do right now. If you don't have charging stations, people will not purchase electric vehicles.
The challenge with a subsidy is that you can't differentiate with a subsidy. Are you giving the money to people who wouldn't have purchased the product without the subsidy, or would they have purchased it anyway? A lot of the money will go to things that would have happened anyway, so I think the infrastructure investment will be much more effective.