The premier difference is cost: an Access 2000 database will fit on a MSDE
2000 or a SQL-Server Express 2005 database and these two solutions are free.
If you know what you are doing, you should be up and running in one or two
days.

However, even with MSDE/SQL-Server, you are likely to have speed problems
over a WAN because Access with linked tables as the frontend is far from
beeing a perfect fit to SQL-Server.  If you go with an Access ADP project
instead, you will solve some of these speed problems but still, you won't
have a perfect fit.

The only way to have a perfect match to a SQL-Server over a WAN at a decent
speed is to use unbound forms and make all the calls to the SQL-Server
yourself by using pass-through queries from VBA or to use some other
technologies like .NET Windows forms or ASP.NET.

In your case, a much simpler solution is probably to use TS or Citrix.  This
is not a perfect solution and it will cost you $$$ but for 10 to 15 users,
you should be fine.  Personally, I don't use TS/Citrix, so I cannot give you
an advice between these two.

When you want to go to WAN, you must know your stuff.