Send With SES uses Tag Based Contact Management while most other ESPs use List Based Contacts. Both ways have their advantages. We like 'Tags' because they allow deeper levels of segmentation of contacts.
Send With SES sends email via your AWS SES Account while others send emails from their own email sending servers.
Send With SES does not charge based on number of contacts/subscribers you have.
There is no reason you should switch to Send With SES if you are happy with your current ESP.
If you have a huge contact/subscriber list but you rarely email them, then switching to Send With SES will definitely save you money.
If you are developing a new app then we highly recommend you to integrate Send With SES. It will save you time and money.