should I use cotoneaster or pyrocantha to espalier up a wall?

Both of them are good for it and will look attractive.
You sound like you've researched a bit already, that's good.
Good to know a bit about what you're planning and doing.

Chaenomeles is another that would do a similar thing - not evergreen though like the others.

Gardeners World a few weeks ago did a great report on how to grow pyracantha up a wall. Joe Swift did it, he visited a house with beautiful gardens and he planted one in the GW garden and will show how to do it, me thinks. It might be worth trying to find it on BBC iplayer. It was a great effect when mature. Pyracantha is quite a quick growing shrub so might be the best bet.

I have just taken a picture of my Garage doorway with Pyracantha and uploaded it. Its not a good picture but does show what you can expect. but watch out for thorns.

Cotoneaster is likely to grow more quickly and does not need support. Either will look good.