1. Everyone I know seems to be going with WES because someone they know did - basically chain reaction since no one knows any better, so we try to go with tried and tested agency.
2. I believe WES is the only one that allows you to take sealed envelope from university and forward it to them. The issue with taking envelopes from Universities is that (as I faced with my UK uni) they refuse to use courier service and use non trackable standard airmail which takes lot of time for delivery. In case of Indian universities, the option to send transcripts is not even available with all of them, like my uni from Hisar doesn't do it so getting docs from them and forwarding to WES is the only option. Does IQAS allow this too?
Anyway these were my reasons personally, but I don't know if it was a good choice because WES doesn't seem all that proactive in replying and not everyone is satisfied with results, nevertheless it is a safe choice.
Also worth noting is that if your documents are pretty straight forward from a recognized university, WES can take as little as a week to do ECA. On the 13 day after all documents were received by WES (all the delay in my case was done by my UK uni), my phsical report was in my hands.