In the end I chose the MGallery Nemetzi because it was only a couple of Euros more than the other Accor hotels in the same little cluster. I like the location of this grouping of brands, all close to the Blaha Lujza tér metro station - it's handy for most things, including the ruin bars

The Nemetzi was pretty good; I'd be happy to return. Was glad of the breakfast, the wifi and the tiny handful of extra British Airways Avios. Rooms were renovated, if a little small. The Mercure does not enjoy good reviews (echoing starflyergold's warning above about non-renovated properties.) Will be trying the Ibis next weekend