IF/Then/Else vs Decode in Select from Dual?

We have a major debate going on.   A developer built logic into PLSQL where he does a select from dual using a decode rather than just using an IF/Then/Else statement in PLSQL.  The question is which one is more efficient. Is there a cost to doing the decode within a select since there is no other reason to have to do a select in this part of the logic?  We are on 10g DB so there should not be any IO doing the select from dual.
What is the best practice here or is there one?