Having just spent the best part of a day re-working a near-complete Flex app to use ArrayCollections instead of XMLListCollections as the data providers for an AdvancedDataGrid I have a simple piece of advice -- make your choice between the two wisely and make the choice before you start.
There's a bug in Flex which means you can't use SummaryRows with data grids that use an XMLListCollection as the data provider. Well, at least it seems like a bug to me. I've read a few people mention it online, but had no response to a question I posted on the topic or seen anything official on the matter.
Here's a demo to show what I mean. Use the buttons to load the same data but with different collection types:
Notice how the donation column only gets summarised for the ArrayCollection and not the XMLListCollection. To me this smells like a bug whereby Flex isn't casting the XML strings to a number properly when leaving it as XML, but it does when it converts the XML to Objects.
Retro-fitting an ArrayCollection can be a pain. If there's even the slimmest chance you you'll need to use SummaryRows then make sure to use an ArrayCollection from the off.