4x5 LF slides only make sense if you have a dedicated film scanner for that particular format, meaning you have to invest into an Imacon X5 or a drum scanner.
6x9 with Fuji lenses and a Nikon Coolscan 9000 are a perfect match for 90% of the tasks. For the ultra large prints on a Lambda or Lightjet you can have the few slides professionally scanned with a drum scanner.
An excellent 6x9 scanned at 4.000 ppi delivers a higher resolution and tonal range (-> higher Dmax) than a 4x5 scanned at 1.200 ppi on a flatbad.