plugin is ready to use immediately
if the Jad site ever goes offline, users aren't screwed (this point can be address with independent hosting)
download is much larger (roughly an extra 1600kb)
upgrading to later versions is slightly more problematic (this point can be addressed with GUI enhancements)
Two points to note:
1. With regards to Hani's post (http://www.intellij.net/forums/message.jspa?messageID=5198071#5198071) any bundled version of Jad would not be in the plugin jar file but open on the filesystem for easy swap-out.
2. I am exceptionally dissatisfied with having to call an external app to decompile something within IntelliJ, so when time and circumstance allow I'll be starting a project to write a 100% Java Java decompiler to make this kind of poll irrelevant. (I will of course google any issues with decompilation first, i can't afford to get sued :) )