Sponsorship an expense or donation for a charitable trust

I have a charitable trust with a 80G Regn. We have paid an amount of Rs. 45,000/- for sponsorship of a marathon organised by another charitable trust. What should be the nature of payment, Expense or Donation?? PS: We have paid the amount wherein the marathon would have a display as hoardings / Trophies, etc of our name in the event. Also whether Service Tax would be levied on the same if above is an expense?