<< ADSL means the download and upload speeds are not the same.
SDSL means the download and upload speeds are the same. >>
That's not entirely true. Some DSL options are technically SDSL, but the upload speed is capped to prevent endusers from running various types of servers.
With true ADSL, such as what is offered by Verizon across most of the country, the upload and download speeds are dependent upon each other. i.e. When uploading, the max download speed will decrease, and vice versa. With SDSL, the upload and download "pipes" are independent of each other; uploading large files will not affect your download speed, and vice versa.
Thoroughly research the options in your area. While a DSL package may have asymmetric speeds, it may still technically be SDSL. When given the choice between 784/384 SDSL or 784/384 ADSL, the SDSL will always give better performance.