Ask YC: Widgets vs Plugins ?

If you're developing a blog add-on, how would you choose between a widget (client side js/flash) and a plugin (server side, powerful) ?

A lot of blog widgets have done very well lately - Mybloglog, Addthis, Feedjit, Whos Amung Us, Entrecard ...

I struggle to recall a plugin which has done as well (Akismet perhaps).

A case in point would be Gravatar (plugin) vs Mybloglog (widget). Gravatar never really took off, Mybloglog is ubiquitous as a sidebar property.