AWS Lightsail Benefits
1. Predictable low price with generous SSD Storage and Network Band included.
2. Simpler interface to manage and provision instances then EC2.
3. Ability to grow into or work alongside the broader AWS Ecosystem.

AWS EC2 Benefits
1. Much larger selection instance types
2. Many more Machine Images available
3. Ability to create your own AMI images
4. More flexable configuration options

Summary
AWS Lightsail is made to compete with the likes of DigitalOcean and Linode. When you take into account the ability to access the broader base of AWS services, most would be best fit with AWS Lightsail.