We have a crawling engine that uses curl and caters to about 400,000 ppl  / month. Although curl supports concurrent downloads, it does not support bandwidth limiting ( only in php 5.4.0 ) which is why my boss wants me to move to aria2. He also says Aria2 is faster ( which seems to be true to me as well )Aria2 would require a large amounts of changes to the system. Right now we have crawling system which does not crawl in a conconrrent manner. Would moving to aria2 be a good decision?
Move from Curl to Aria2?
php
After running quiet a few test we found aria and curl/php/apache are running at almost the same speed. We decided to stick to curl as it has a php library libcurl. We are not sure about bandwidth limiting but are working on it. Changing our current code to crawl multiple pages simultaneously was not such a big hassle as of curl. Although there have been some things we need to work around e.g. https://stackoverflow.com/questions/8878546/curl-multi-crawling-issues