Worked there and been to both, seen Boca v river at both, for me if you have the chance to go to only one go to Boca, the area is real with burnt out Cortinas and equivalent, the bars are much better and there's more history whereas river is in a snotty neighbourhood with Land Rover / Porsche garages. And they play in dung colours. Check that away fans aren't banned first, recent games have been subject to a 'no away fans' ruling. The hatred is still there but there's nobody to hate ! And it's a great city.