I work for an ISP, we sell all the connection types. ADSL, cable, T1, Fibre, Radio and good old 56K. I can clear up a few things.
Yes for home use cable will usually always be faster for downloads as it has a nice "burst" option. Bursting is a temporary increase in download speeds.
Yes the server you connect to also has to have a fast connection too as you can only download as fast as it uploads.
Yes cable users will experience a slow down when more users on their "loop" jump on the Internet. We call this "After School Syndrome" when all the kids come home at 3:00pm and start up their Halo servers.
ADSL is a direct connection between you and your ISP and so speeds are more consistent.
ADSL speed is determined by what your are paying for, line quality and inside wiring issues(ISW) and how far your house is from the nearest Central Office(CO). The ISP can test your line quality and loop length remotely and set your speed accordingly to get best stable speeds.
ADSL is USUALLY piggy-backed on a telephone service but any telephones, fax machine, alarm systems, Interac machines or other analog devices MUST be
behind an ADSL filter or the ADSL signal will be crappy and intermittent. POTS filters are usually installed at the DMARC(the box outside your house) and all analog devices must chain off the "phone" side of the POTS filter. Inline filters sit on the phone line of every analog device. If you do not filter out your phones and other analog devices your ADSL will be intermittent.
ADSL that is not on a telephone service is call "Naked ADSL" or Dry Copper ADSL" the signal still comes over a pair copper wires like a phone line but their is no dial-tone and no telephone service. Dry copper is cheaper as you do not pay for phone service and their is NO CHANCE of an analog device interfering with the signal. Very nice.
The length of telephone wire from the ADSL modem to your wall jack should NEVER be longer than 6 feet, this wire is unshielded and if it is long the signal will be degraded. This is often overlooked.
Upload speeds will rarely be above 650 kbps for cable or ADSL.
When doing speedtests do NOT choose the small size files as the test is over as soon as it is started and thus not accurate. For download testing choose at least 3.0mb file size unless your connection is really slow.
Any accurate speedtest will take at least 45 seconds to complete.
Speeds Degrading? Reboot your modem, routers, switches and hubs, but do so one at a time. If your connection is degraded, reboot your modem. If that does not solve it then reboot your next device, usually a router. If after rebooting the router things improve then you know the router is at fault and not the modem. Rebooting everything at once you will be chasing your tail trying to ID the failing device.
Do you have to reboot your cable or ADSL modem more than once a month? Call your ISP and demand a new one.
That's it for now. Any questions please let me know.
Snaz.