If i have DSL and my computer is connected to the modem/routher by USB cable.

ive alwaysused ethernet and its worked great, but im not an expert

Ethernet Cable will give you better speed.Crossover cable to choose.

It's a known fact that connection through ethernet cable will give you more speed, while being more stable compared with USB connection. However, on the PC - modem circuit, you can use USB connection without any problems. As you probably know the future will give us USB 3.0 with an extraordinary transfer speed. To answer your question, i recommend connection through ethernet cable. The cable must be straight, crossover cable is used on PC to PC connection. Good luck and keep up the good work here!

First, I've done both. The connection from computer to modem has no bearing on your modem to internet connection per say, at least not one you will visually note a difference with. Ethernet is an older, more stable technology than USB would be. USB is stable but from my experiences I prefer the older technology of Ethernet cable due to the fact that it is more proven in age of technology than USB is. The communication between a computer and a component connected to it can only go as fast as the computer CPU can process it. If you have a 356 computer with Ethernet connection it is going to process at the speed at which it is capable of doing physically. If you have a dual core processor it too will process the communications between a connected device at the speed it is physically capable of and no more than that. Your internet speed depends on where you are on the network, what type of network you have with your ISP and what your ISP provides you with services wise. If you are close to a hub on the network (physical hub by the way not a virtual one) then your speed is going to be more because the shortest distance in cabling sends the packets of information being communicated "faster" than someone twice the distance from the hub than you are. Basically the closer you are the better it is. The modem itself can handle only according to it's specs, if the modem is old, has a lower tolerance then you are getting what that modem can handle. If you have top of the line and it's set up properly (ie: your router settings can slow you down depending on the settings) you will see the maximum benefit of your ISP. As for ISP - if they cap their bandwidth (a common occurance to battle P2P users) then there is nothing you can do at peek hours when the cap is in place. It's also called bit cap (restricting the bit rate or ability to send bits of information at higher speeds). Many ISP's do this not only for P2P users but also do this to restrict perceived bandwidth abusers (ie: P2P users, excessive upload and downloads, even exceeding upload\download quotas on your services on a monthly basis). It's mostly to battle P2P users mind you. In the short answer, I'd recommend the Ethernet for it's stability and time tested accuracy rather than the USB. I'd also recommend talking to your ISP about what their different services are and getting them to help you decide which is best suited for your needs as well.