What are the advantages and disadvantages of being an office incumbent in Washington, DC? Which incumbent is more likely to win an election—congressional or presidential incumbent? Why?

There are many advantages to being an incumbent in Washington D.C.  This explains why the turnover rate of Congress is so small even though public approval of Congress is at an all-time low.  Many voters go to the polls and vote for familiar names--this gives the incumbent an advantage.  Incumbents also receive more money from the national party conventions.  Incumbents also have the experience of serving on Congressional committees.  It is also easier for a presidential incumbent to ask for donations; there is a space in one's income tax returns where one can make a donation to the president's campaign.  
There are also numerous disadvantages to being an incumbent.  A party that has fallen out of favor with the public, such as the Democrats in 2010, might lose congressional seats because of the public's overall dissatisfaction.  Political activists can also target incumbents as being ineffective, and social media allows for this news to spread before the member of Congress can explain his/her stance properly.  Also, scandals are more magnified on Capitol Hill than they would be in the private sector.  
It is easier for a presidential incumbent to be reelected.  If one looks at the track record, there has only been a single one-term president in the last thirty years, George H.W. Bush, who was defeated in 1992.  Presidents have more resources and more staff and other key party members who will give speeches on their behalf. Many states have fallen into patterns of either giving their votes to Republicans or Democrats consistently, leaving a handful of battleground states on which a sitting president can focus his/her resources.  Presidents receive a great deal of credit when things are going well for the country, and it seems as though it takes a national economic crisis or scandal for the party in power to be replaced in the Oval Office.