Robben ford is much more rawer (is that a word?) then Eric Johnson. Robben takes much more chances and improvises his solos, never plays the same thing twice.
Eric composes everything, including his solos, is very much a perfectionist (he admits this, spending years on one album) and relies much more on effects.
Who's better? I guess it depends on what type of night Robben is having.
Personally I think there both great, but I find Robbens playing a bit more exciting and unprodictable.