NAT Gateway support bursts up to 10 Gbps[1] and t2.small something like 125-200 Mbps [2]. For production use cases this can make a huge difference.
[1] http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-na...
[2] https://stackoverflow.com/questions/18507405/ec2-instance-ty...