.txt is an ancient and extremely well-established extension, and so will not cause any trouble anywhere—it’ll be mapped to text/plain in all standard servers, out of the box.

.text is not a common extension. Some things know that it’s text/plain (my Arch Linux /etc/mime.types and /etc/nginx/mime.types both do), but I expect some common server software won’t handle it properly out of the box (haven’t checked beyond nginx’s mime.types), and common OSes won’t have a handler for .text files set up (Windows, for example, comes with .text set to PerceivedType text like .txt has, so that it’ll suggest the right sort of apps to open it, but it’s still not hooked up to any app by default, unlike .txt which is “Text Document”).