That’s not really apples to apples due to the storage difference but:
For £70 go with the 11, if it’s nearer £100 or more go with the XR is my personal view. It’s a newer phone with a years more support, better battery life and faster processor.
That said you could get a brand new XR for £350 from O2 on Black Friday so £500 for a used one feels a bit steep.
Seems to be the going rate at the minute. 128gb of the 11 is about 150 quid more. Don't need really need anything powerful.