It really comes down to who takes the beating.
Before the current method of mounting the headphone jack floating from the primary circuit board - they were the most frequent points of failure. If you tugged too hard on the cord, you'd rip the connector off the board and/or damage the board. In that scenario, you'd likely be safer using an angled connector, as it limits the amount of leverage applied to the jack. Most consumer devices these days have "floating" jacks - so even if you twist them, or flex too hard - they do not experience / transmit mechanical damage.
But, when inserting / removing the plug - angled plugs tend to fail more often because people don't always grasp the base - but instead tug on the cord.
In short - If the device it in your pocket, an angled plug usually is safer than a straight. But there a lot more variables at play form how you treat things, and the quality of whatever it is you buy.
I'm an angled person, myself.
