- Attach if you want to archive a snapshot of the file and not keep it updated
The advantage of attached files is that they will go wherever the map goes. Links will go out of range if you send the map to someone else.
If the links are going to work on multiple devices then the URL needs to be same from each device. If you are using a Dropbox folder on multiple PCs and maybe Macs the simple relative file links will work. e.g. ..\..\my.doc These do not work on the iPad or iPhone.
You can get the shared URL public link from Dropbox and link to the web version of the file. I couldn't drag and drop these from the Dropbox web page to MindManager. However when first I tried to use these links on Mindjet iPad it had added some extra http's and it didn't work the first time.
The iPad and iPhone apps will not open attached files in maps
If you can open the map on multiple devices, who is the master? Dropbox noted that I was editing the map below from two devices and created a new copy for one the devices with the device name included in the file name: "Dropbox Test (2740p's conflicted copy 2014-09-26)"
Try opening this map and see which links and attachments work.
Here are some previous thoughts about Dropbox and MindManager.