What is better: a beautiful letter or no formatting?

Question

Email

There was a need to send letters to clients with an invitation to our events.
1. I can’t figure out how best to do it: a beautifully designed letter with modern flat graphics is often perceived as spam and, despite being adapted, it still looks very different in different email clients and on different devices. Or use text without formatting - maybe it turns out to be more boring, but the text looks the same and reacts to it better.
2. Write the essence in 5/6 sentences and, upon request, send an invitation with the program of the event or immediately send it with the program (the message is stretched to 40/50 sentences). There are those who say that it is better to get the program right away in order to understand whether it is interesting or not, and there are also those who do not read long letters. How to be?

Answer 1

Try a/b testing. Choose an audience, send out half of the letters with graphics, and the other half without it. View the conversion and draw conclusions.
Hardly anyone can answer your question. The results are very specific.