- Google Maps Flex API
One is the ability to view Open street maps. In case you haven't heard open street maps are the same for GIS that Wikipedia is for textual information.
Check out a cool map of Lahore (Pakistan) using UMap :
Plus you can customize the display logo at the bottom to your own company logo (for OSM) !
Also Yahoo maps seem to be more detailed for Remote locations than google maps (Virtual earth is the worst for third world countries).
If you want Google maps, go with google maps. If your client specifically demands greater detailed maps or yahoo / virtual earth / open street maps ... go with UMap... even if the demand is only yahoo maps . UMap gives you more than the default yahoo control.
Google maps seems to be more hot nowadays so I will stick with it. ( Everyone has heard of it :) )
Also google maps supports the Adobe AIR Runtime (still experimental .. but stable). And it will soon have support for Google Street ( Open street map type maps by google )
P.S: both support polygon drawing (necessary for azimuth based GSM cell representation). So you can always swap between the two, as long as you abstract the APIs which you should be doing anyways :).
Google Flex Maps