I find agent mode much more powerful as it can search your code base for further reference and even has access to other systems (I haven't seen exactly what is the other level of access, I'm guessing it isn't full access to the web but it can access certain only info repositories). I do find it sometime a little over eager to do instead of explain, so Ask mode is still useful when you want explanations. It also appears that agent has the search capabilities while ask does not, but it might also be something recently added to both and I just don't recall it from being in ask mode as I'm use to the past when it wasn't present.