Non-silenced unless it has to be. If a silenced weapon needs to be carried it's better to use a handgun calibre weapon (9*19mm, 9*18mm or 0.45 calibre). Rifle suppressors are ineffective, at best. (Generally generating a similar level of sound to a car door being slammed)
In addition, subsonic 5,56 or 5,45 ammo severely lacks stopping power, almost every modern military has vests that'll stop that. Only very developed nations have vests that'll stop a standard round, and even these are standard issue for most troops due to their weight and general clunkyness.
The obvious exception to this is the new Russian 9*39mm round, used in the AS VAL and VSS Vintorez rifles (and others, though these are best silenced ones), which maintains high subsonic stopping power due to it's size and design.
Stealth operations by no means should rely on silenced weapons, as they still generate a visible flash. This is extremely noticeable during night-time as well. (IIRC, during the night, the human eye can pick out a single candle 5km away)
I'd also like to state that if you didn't know about suppressed weapons needing subsonic ammunition you probably shouldn't argue with this.
And now, to debunk the flawed arguments everyone else has made!
Anyway, pretty much every army in the world uses the sneaky approach too, it's sensible battle planning. Apart from the USA, of course. The USA seems to love being loud and proud...
I'mma call bullshit. Out of the major modern armies (The US, UK, Germany, France, Israel, India, Russia and China) the US, if anything, use the most silenced weapons per head. I don't have anything to back this, and it's possible the Russian's have more, but they don't use them as much. The Russian Doctrine focuses heavily on high-mobility, well armed and armoured task force tactics, with specialist operations being used for peacekeeping afterwards.
Silenced guns.
They make Pew pew noises.
Nopers! They make "Thunk thunk" and "click click" noises. Watch this:
http://www.youtube.com/watch?v=mF8d0XcWSVw
and this:
http://www.youtube.com/watch?v=vt7O1-ZO-Rg