The advantage to using a lookup table is maintainability. As pointed out, there are lots of ways to create such a table and lots of places it can be kept. Personally, I'm liking the new file backing for global/flow "variables" - I tend to have a simple creation flow which is very easy to maintain but I know that the data is always there and is easy to view in the admin interface.
JSONata is also easy to follow for simple transforms such as the one mentioned.
As Dave says, all depends on your experience and needs.
The easiest structure of all would be an object keyed on the number.
"140": "Automatic Mode",
Then you can do the lookup with a simple (function example, JSONata even simpler):
msg.payload = msg.payload + varName[msg.payload]