Your best waiter suggests a topping when there is time. The kiosk suggests one at 12:15 on a Saturday too. Measured on real orders, that one habit is where the money is.

Guests do not pick pricier items. The average paid topping costs about the same at both channels (€3.97 vs €3.56). The difference is how often anyone gets asked.
Peak hour is when the till goes quiet. With a queue building, staff stop suggesting extras. The kiosk shows the topping screen to every guest, including at 12:15 on a Saturday.
Browsing without a queue behind you. Guests take their time, read the pictures, and add the extra egg they would have been too rushed to order out loud.
Both run the same Self Station app, with the same menu, the same languages and the same upsell screens. The only question is how much counter space and budget you want to give it.
Self Station runs on a standard Android tablet. Lock one into a counter stand and the whole station fits in about 20 cm of counter — no floor space, no cabling work, and you can reuse a tablet you already own.
The 21.5″ floor kiosk is the full station: bigger screen, card terminal, receipt printer and QR scanner in one bolted-down unit. It carries the queue when the counter is busy.