You obviously feel as though you should fess up. So it is better to do so as early as possible. If you don’t it will trouble you for days or months, and in the end you will have to face the music. Fess up now and avoid all the stress of waiting to be found out.
On a similar vein, If you have messed up, and apologies from you are in order, go out of your way to do that asap. Again, it will save yourself a lot of grief. A quick apology means the incident is more quickly forgotten (got over).