Ideally I'd like a 2.8 inch and it to be touch screen. But most of all I'd rather it work on both boards without much hassle.
Well any display can work on both of the boards if you can adjust the voltage levels right there is a display which was made for old Nokia phones and it is called nokia-5110-3310 and is dirt cheap display and it can be used with both Arduino or raspberry pi or any other board which support the protocol here is a link for quick reference.
adafruit has a resistive touch screen which is 2.8" and works on most boards. Its 30 dollars and works well. It has instructions and all and libraries which you can use. Plug it into a breadboard, connect the jumpers and you're all set.
on the other hand, Elegoo has made a very similar product, which can also be used on most boards if you plug it into a breadboard, but it is specially designed for the UNO. You can plug it straight into the UNO board, without the need of breadboards. This comes with a instructional DVD and is only 12 dollars on Amazon. It is also a resistive screen and basically the same product.