Rain "jackets" or parkas are best used as outerwear for a windy, cool day. I recommend a bright color to use as a wind shell. Always try to get one that has "pit zips" so you can open these vents in your armpits for extra ventilation. It helps - a lot.
As a general rule, and as most of the comments above correctly state, a backpacking poncho offers better protection against the rain. Your rucksack should go UNDER the poncho. Rucksack rain covers do not prevent water from getting to the area against your back, and infiltrating your rucksack from there.
I have adopted several practices to overcome some inherent problems with ponchos:
1. Wear a baseball style cap to keep rain off your eyeglasses, and to allow the hood to fit closely arpound your head. If you wear a broad brimmed hat, water will find it's way in, and down your back.
2. Have an extra pair of boot laces, or a lightweight bungee cord, to wrap around your waist, to hold the poncho closer to you to prevent the poncho from billowing all about. Everyone seems to complain about this poncho attribute, but the simple cord or elastic (bungee) cord is an easy fix. When you reach the day's destination, you can string this up to create extra drying space for your wet clothing.
Hope this helps.
Tom