Is the electrophile in sulfonation of benzene SO3 or SO3H+? What's the electrophile in sulfonation of benzene? Is it $\ce{SO3}$ or $\ce{SO3H+}$? 

According to what my professor taught me, both $\ce{SO3}$ and $\ce{SO3H+}$ may be the electrophile involved - and mechanisms with both species can be successfully written. 

I'm looking for some source(s) that confirms the existence of both electrophiles in sulfonation reaction of benzene.
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.