I keep referring to this article, but for me it sums up the situation perfectly:
http://www.aerobatics.org.uk/Journal_Nov_07.pdf#page=39
Make your own mind up. With specific reference to the Decathlon, if possible, select an enclosure type which can act as a subsitute for the existing seat cushions - if you can remove the seat back, a backpack type, or if the seat base cushion can be removed, a seat pack. I use an Irvin GQ seat pack in the Yak, and an Irvin Silhouette in the Pitts & Extra - although a lot of people I fly with use the National backpacks. If possible, get hold of a few different enclosure types and makes and try them out sitting in the aircraft - what works perfectly for one person can mean mild agony from buckles poking into bits of someone else.
However, as the article above shows, so much of the effectiveness of emergency equipment is the attitude you have towards its use.