The Old UI is our old User Interface, which is what our clients originally used. One way to tell whether you're using this interface is to look at the http address. If it begins with app.squirrelstreet.com, you're using the old UI. In the old UI, you can't do things like customising your column view or necessarily add the credit card number used to pay for a purchase.
The Digit UI is our latest User Interface, and we're quite excited about it! You can do nifty things like code a receipt in two clicks (for those with accounting software) and we believe it's more intuitive to use. You can access it with this https address: https://digit.squirrelstreet.com