This may be a touchy topic because what some see an advantage others might see as a disadvantage but it's nice to weigh out the options.
-local support from your actual carrier and more willingness to work with you if there's issues with service and device (I recently just received a MASSIVE discount towards any new device even though I am far from early upgrade, due to multiple swap-outs of my phone)
-insurance on your device
-depending on how the merger concludes you may see additional cell coverage through added bands and roaming
-the look and feel of the phone may change (I.e. the 4 capacatative touch sensors that the U.S. is accustomed to)
-upgraded features or additions to the stock user interface for those that don't root and modify. Example: the Optimus 2x vs the G2x added HSPA+, music player and stock "vanilla" u. I.
-if there's issues that turn out to be hardware issues they are generally found and fixed by the time it rolls out over here
-you can use an early upgrade if elligible and save money. Your personal option.
These are all the things I can think of at the moment