I've been using MoneyGram for a long time now and I've had very few problems. The rate is almost always better than Western Union (after fees, for transfers from the US, at least), and sometimes it's substantially better. Transfers to Banco Nación take a little over an hour, also a plus (for me, it's always a little unsettling to send money to Argentina; the faster that I know that everything went ok, the better).
As far as I've seen, when people have had problems with transfers, they have always been resolved one way or another (either the money arrives late or the transfer is cancelled and the funds returned). I've had these problems twice; once about a year ago for a week, and late last year for a couple of days. Otherwise, smooth sailing.
I would certainly recommend MoneyGram. I think it's good (for all of us) that Western Union should have a strong competitor for the business in Argentina.