Welcome to the Accor forum ajamieson

Well from a brand perspective: ibis is a generic cookie cutter brand with basic but clean rooms. Emphasis is on basic. Novotel is something akin to Holiday Inn, again they tend to be quite similar. Mercure on the other hand are more unique but can vary in renovation state. Modern recently renovated Mercure can be great, have a bit of character and would be my choice, especially if keenly priced. Not renovated ones can be bit dodgy.

I'd recommend doing a search for each brand+Budapest, there should be some reviews in the "Please post your Le Club Elite experiences" threads.

There are some good reports of the MGallery in Budapest and the Sofitel can price well at times.