That depends on the type of application, schema, and number of records you
handle.
if you have a simple schema, and few records, then you probably will not see
much advantage.
if you application will grow to 100"s thousand of records, then it is best
to start with SQL since there is a time cost to covert.

Dis-advantages- again depend on the application. Learning curve is the
biggest.

SQL Server can handle a lot more users / records / and has
more features, e.g. triggers, etc ...

You have to buy the lisence for SQL Sever (not the MSDE)
while JET Database Engine comes with Access.  

Also, you will need to do a bit of learning if you want to
use SQL Server back-end and spend some effort in
converting the front-end to work effciently with the SQL
Server back-end.

HTH
Van T. Dinh
MVP (Access)