Is the electrophile in sulfonation of benzene SO3 or SO3H+?
From Clayden, under the heading "Sulfonation of Benzene" (electrophilic aromatic substitution): 


  The cationic intermediate can
  also be formed by the protonation
  of sulfur trioxide, $\ce{SO3}$, and
  another way to do sulfonations is
  to use concentrated sulfuric acid
  with $\ce{SO3}$ added. These solutions
  have the industrial name oleum.
  It is possible that the sulfonating
  agent in all these reactions is not
  protonated $\ce{SO3}$ but $\ce{SO3}$ itself.




From March's $7^{th}$ edition, under Sulfur electrophiles:


  Sulfonation is often accomplished with concentrated sulfuric acid, but it can also be
  done with fuming sulfuric acid, $\ce{SO3}, \ce{ClSO2OH}, \ce{ClSO2NMe2/In(OTf)3}$ or other reagents. 
  
  Sulfur trioxide reacts much more rapidly
  than sulfuric acid with benzene it is nearly instantaneous
  
  A great deal of work has been done on the mechanism, chiefly by Cerfontain
  and co-workers. Mechanistic study is made difficult by the complicated nature of
  the solutions. Indications are that the electrophile varies with the reagent, though
  $\ce{SO3}$ is involved in all cases, either free or combined with a carrier. In aqueous
  $\ce{H2SO4}$ solutions, the electrophile is thought to be $\ce{H3SO4+}$
  (or a combination of
  $\ce{H2SO4}$ and $\ce{H3O+}$) at concentrations below $\sim 80–85%$ $\ce{H2SO4}$, and $\ce{H2S2O7}$ (or a
  combination of $\ce{H2SO4}$ and $\ce{SO3}$) at concentrations higher than this. (the changeover
  point varies with the substrate)
  
  In fuming sulfuric acid ($\ce{H2SO4}$ containing
  excess $\ce{SO3}$), the electrophile is thought to be $\ce{H3S2O7+}$
  (protonated $\ce{H2S2O7}$) up to $\sim 104% \ce{H2SO4}$ and $\ce{H2S4O13} (\ce{H2SO4+ 3SO3})$ beyond this concentration.
  
  Finally, when pure $\ce{SO3}$ is the reagent in aprotic solvents, $\ce{SO3}$ itself is the actual
  electrophile. Free $\ce{SO3}$ is the most reactive of all these species, so that attack
  here is generally fast and a subsequent step is usually rate determining, at least
  in some solvents.