Fosphenytoin is more water soluble and does not require dilution with propylene glycol and alcohol. Consequently, this drug lacks phenytoin's adverse effects such as hypotension, cardiac arrhythmias, and infusion site reactions; and can be given at a much higher IV infusion rate 150 mg/min (relative to 50 mg/min) or, if necessary, even given IM. The main disadvantage used to be cost but Fosphenytoin is now available generically.
Source
Hung, O. and Shih, R. "Antiepileptic Drugs: The Old and the New" Emerg Med Clin N Am. 2011.