Well, above around 1200NM, the Ejets are "too small", so there is no question.
Below, the question is much more interesting. And the answer is "it depends". It depends on the opposition. It depends on your overall strategy. It depends on your schedule opportunities.
I'm partial, because I'm currently flying both Ejets & 737 in current GW3. They are a good complement. Ejets can be efficient on smaller routes, trouncing opposition with frequency. OTOH, on bigger routes, they suffer from slot scarcity. And the lower cost-per-seat of the 737 makes it a good choice. The longer the route, the better.
In other words, it depends from your current GW, from your base airports, from opposition. From your strategy, also. What are other route opportunities? what would fill them? Are you in a country filled with opposition like the USA? Are you alone in a middle_sized country like Jordan? What is the average size & distance of your potential routes? Can you afford to wait to the C919 or the MRJ? There is no easy answer. You have to analyze your situation, guess what opponents will do, and take a risk. That guy who was flying 737 short routes from Faro did stand no chance against my Ejets. He BK's quickly. But I didn't attack his few longer routes. I was not strong enough(yet).
And a strategy is not static. In previous GW3, I had begun with a lot of CRJs. I was planning to upgrade my fleet to bigger CRJs. Then, in the early 2010s, I noticed that prices of A148 had plummeted. I could buy 3 of them for one single CRJ700. I completely changed my strategy, and it's been a very successful move. I got rid of nearly 200 CRJs within 10 years. Because of the specifics of that game, it was overall cheaper to fly A148s, the gain on the owning cost was more than enough to offset all other costs. But I can't do the same in current GW3 : A148 is no more built, and I'm having a base in London City where it cannot fly anyways. Hence the Ejets.
Your question is interesting, but never forget that you have to adapt everything that you read in answers to your own position.