From different perspectives I would say that Orlando's answer is good in relation to the general idea of the internet. Gadeliow's answer well describes the terms in reference to a server-client relationship.
(not giving my opinion on answers, giving credit where it's due.)
But some software tracks up/downlink in reference to the interface. So download would be incoming, and upload would be outgoing. I have a switch/router that tracks like that. such a headache.
For choosing up/downlink to test, you need to determine what your server is being used for. What kind of load do you want it to endure?
In every case I can think of though, the downlink from the server to the clients will be under more strain. Even a file server will be used to download hosted files more frequently than they are uploaded.