There are really only 2 things that will have major impact on the upload speed they are
"Upload Connection or Upload Bandwidth"
100mbit is the standard obviously. 1gbit (1000mbit) is obviously much better but obviously more pricey.
"Users per Server"
This is probably the most important thing. Dont forget unless you are paying hundreds of dollars for a dedicated server you are always sharing the connection with other people.
The lower this number the better - many usually share it between 8-11 users. Also some of the less reputable providers will lie as to how many users are sharing the server.
Gamestuff.ca are a good example they usually limit there servers to 4 users or less.