The difference is one got elected and - if he last that long will get turfed out under the '13yrs of tory misrule' Rule.The difference between Starmer and Johnson is that Johnson is authentic and Starmer isn't. Johnson represents a class - a very powerful dominant class; Starmer is in thrall to a series of focus groups.
Starmer just has to sit tight and avoid O.Gs
How long he lasts once elected is another question