Being point of sale, it would be best to build a GUI app. Airports especially "still" use non-GUI apps (possibly due to the cost of upgrading), but it'd be far nicer for your end users to provide a clean and clear way to work with your application.

If it were to be a web app you could (for example) use a PDF library to produce receipts and handling printing that way. In this case, you could also provide a simple and clear way to customise the receipts.

Certainly the advantage of it being a web app is that you wouldn't specifically have a requirement for Linux (if for example they already have a set of workstations) or installed applications. This would mean that implementation would be far easier and say, managers could quickly pull up the POS system for a specific client in the back office.

The only issue I can see with a web app solution is interfacing with hardware. You would either need to forgoe the possibility of hardware, or work out a way to interface them.

I think there are countless possibilities, most of which will be rather dependent on the local businesses. They seem to have provided you with a lot of breathing space, so it may well be that many solutions will fit and you are free to choose which would work best for you.