If you can afford a high quality breathable waterproof jacket then that is probably the best option.
Having said that, we found that wearing heavy duty ex-military ponchos kept the rain out, gave us freedom of movement, kept our rucksacks dry and felt very secure.
Waterproof ponchos present the wearer with three issues though.
- They are not breathable so perspiration will gather on the inside
- Because of their length, when climbing rocky outcrops it's
possible to step on the leading edge of the poncho causing tripping.
Likewise the leading edge sometimes obscures the ground making it
difficult to determine where to place the feet.
- In high winds they can balloon which can restrict visibility.
Our ponchos came with waist ties to deter ballooning and performed excellently during the hike which at £20 each mail order seems good value.