You really need to try them both... I think your questions will be answered best that way.
I use both. Advantages for iDisk is that you get more room to work with. But it's not free. Though iDisk does more things.... so if you get iDisk for the other features, then it may be worth it.
iDisk can also keep iCal synched (so both your desktop and mobile systems show the same calendars) as well as your Safari bookmarks. Add a bookmark to Safari on your mobile and it will appear on desktop system (of course these options and others are optional). However, I do sometimes run into synching issues with documents. This only happens when I am working on a document on both systems, and/or open the document on the 2nd system before the 1st system has had a chance to fully upload to the iDisk servers. I only keep a sub-set of documents synched through iDisk.
I use iDisk for the bulk of my synching needs, personally. Dropbox I use to synch with others because it is really easy for friends/family/colleagues to set up an account. Also, every person who sets up a Dropbox account through your invitation adds some space to your account. Dropbox seems to work a little more smoothly than iDisk, but I don't use Dropbox as much as iDisk. So far I have encountered no issues with it.
Both packages work by copying documents to a server and holding it there. You can choose to treat iDisk as an external HD. Documents are held there, but not on your local HD. Local applications can access, save, etc the documents on iDisk... however, there are two caveats. One is that uploading a changed document can take time, and the whole system can bog down while doing so. Big documents can take 5, 10 seconds or more depending on size and internet connections. And the 2nd caveat is that iDisk in this setup is only available when you have an internet connection. I have my desktop set like this, and put up with the delays because it saves on local HD space. I may change this soon... I have lots of HD space.
The second way to set iDisk is to have a local copy of iDisk on your HD. I have the laptop set this way. You access the iDisk folders as if they are local folders (well, they are - I guess) so speed is not an issue. The local folders are then copied to the iDisk servers when you have an internet connection. So... if you don't have an internet connection you still have all of your iDisk content handy and available. Soon as you reconnect the iDisk servers are updated, and then those changes are sent down to whatever other computers you have synched.
I'm pretty sure that Dropbox works like this second iDisk scenario. Local copy, and then changes copied to their servers.
One of the things I like about Dropbox is the ability to share just one folder with someone. In the Dropbox model this other person (or persons) can set up this folder on their system - as a folder or directory depending on their OS choice - and access it as a local folder. Changes made by one person are propagated to everyone else's folder as well. Works really well as a collaboration tool.
iDisk folders (whether local or external) are accessed through the iDisk icon found in the 'Devices' area of the Finder. You can set up a folder structure there to suit your needs (though at least once upon a time it was better to put all your document folders within the iDisk 'Document' folder.... I forget why now. Once you have set up your folders within iDisk you can drag them to your own folders as shortcuts, or put them into the Finder sidebar.
Dropbox, iirc, puts a 'Dropbox' folder on your system, which you can place where you want (perhaps there are some restrictions?). You then create a folder structure within it to suit your needs.
Hope this helps......
