Until recently I used to be a big convert to the Nth Power xt wit the double pullies for my RS4's as manually dowenhauling would lead to me being able to slice washer of my freckle I was straining that hard.

Recently I've gone over to the NP extension with the angled pullies, can now downhaul just with a cleat with a minimum of fuss and strain haven't used the nth since.

In my opinion you cant beat the Chinook ones ,

One thing that helps allot is knowing how to loop the ropes CORRECTLY!!

I didnt know how then i took the time to learn one day and it is so much easier to downhaul , none of the ropes rub or frey against eachother... its great!!

Another thing to think about is the hole where the pin goes in,
I had an aeron extension and the hole wore out pretty quickly. ( altho the shop where i got it reckons that it doesnt happen often)

The new aeron Extensions have a metal insert to stop the plastic wearing out , but i dont think its much better.