"Es sólo una vieja costumbre."
Translation:It is just an old habit.
This is a pretty good explanation: http://spanish.about.com/cs/grammar/a/whereadjective.htm
They are synonyms. You can freely exchange them, but using solamente generates a bit more intensity due to being a longer word. :)
Here's an RAE link. The RAE now even discourages the use of the accent and recommends to just write solo, but if it's ambiguous, you still have the option to place the accent.