If you leave it, there is a possibility it could set some small hard black seeds which you can sow next spring to increase your stock. Canna will often flower from an early sowing in the same year, if it's a good season.
If you cut the flower off, it's unlikely to flower again this year but in theory helps the plant build up its strength. Cosmetically it will look better for the rest of the summer season.
When sowing, the seeds are extremely hard and soaking might help. The batch I sowed in March this year in a pot in the greenhouse are still germinating, while the first ones that came up are now two feet high! A lot of warmth and moisture seems to help germination. Seed don't necessarily come true but it's worth trying.