I've reviewed my old bills and my average data usage is 1.3GB on 3G. One month saw 2.2GB and one saw 3.3GB.
Now, keeping in mind that 4G is roughly 10-20X faster, I know I will start with the 2GB plan and closely monitor my data usage, but I have come to peace with the fact that I will likely and happily pay for a 5GB or higher plan.
The poor 3G speeds are ultimately what have held me back from using more data. Now that I will be able to browse, stream, and game more effortlessly, I will, and I will pay for the service.