if your looking at doing anything professional, fibre is the only choice. ADSL suffers many problems. Tcp doesn't really like or understand asymmetric connections.
I've diagnosed many problems with ADSL physical layer. Here in the UK we get slower sync speeds at night due to interference from street lighting!. This causes errors, retransmits , high latencies and lower throughputs. Depending on the length of your adsl line to the exchange will effect bandwidth and day to day reliability.
I've had customers that loose half their speed during wet weather, somewhere in the path of the line, the copper cable is exposed, and is being shorted by water!!! Sure they can move you to another line, but it means downtime. Usually the copper cabling is barely enough to support a phone call.
ADSL is okay for domestic broadband, however it's not the solution for business networks supporting services such as vpn's. If you have no alternative to adsl, then try for sdsl,it's symmetric nature will make things at the tcp layer run a bit smoother.
If your ADSL line is syncing at a high rate, force it to sync at a lower rate, thus reducing thenumber of error's you'll be getting.