If you are deciding between Dialogflow ES and Dialogflow CX, here are some rules of thumb you can use.
I recommend using Dialogflow ES in the following cases.
You are building a large FAQ chatbot
A large FAQ chatbot is not likely to answer follow up questions. Usually these provide links to other resources etc.
In fact, if your chatbot can benefit from my tool which allows you to type your training phrases into a spreadsheet and bulk convert them into a Dialogflow agent ZIP file, you should just use Dialogflow ES.
Your bot does not have complex multi-turn conversations
You are on a limited budget
If you are on a tight budget, Dialogflow ES is easily the better choice. It has a very generous free tier.
But you need to balance this with the fact that it is harder to use ES to construct complex conversation flows as I have mentioned before.
You need to integrate your bot with external apps and services
Dialogflow ES has been around for much longer than CX. As a result, you will find that there are more external apps and services which integrate with Dialogflow ES but do not integrate with Dialogflow CX.
For example, Zoho SalesIQ has a 1-click integration with Dialogflow ES but does not (yet) have a 1-click integration with Dialogflow CX.
Also check out the companion article: When to use Dialogflow CX