The Silver Chair is what is obviously being alluded to as the most logical next film at the close of VOTDT by the inclusion of mentioning Jill.
However, if it comes down to what type of financing they are able to get and think it would be more cost effective to do Magicians Nephew then so be it.
I really feel that FOX fumbled the marketing ball on VOTDT. Thankfully it got some domestic legs that have carried across $100m and if the international box office hadn't responded we wouldn't even be having this either/or discussion.
Whichever film gets made next needs to have a real strong publicity push that VOTDT didn't get.
I don't think MN is going to be more cost effective compared to SC. SC could easily subsitute Ireland for upper Narnia. MN does not really have filmable locations so those will have to rely on effects and CG. This costs lots of money. Just some scenes/places that need effects off the top of my head:
- warping to and from the wood beyond the world
-some aspects of the wood beyond the world
- charn, especially shots of the sun and the destroyed world.
- Jadis bending the lamppost
- the creation of Narnia
-Aslan
- The lamppost growing
- Animals coming from the bubbling land
- Talking animals
- Strawberry getting wings
-Flying on strawberry
-the toffy tree
- The tree growing from the apple
- possibly the glowing apple given to Digory's mother
- the core growing quickly in a tree
- the wardrobe, as I'm assuming that they will have it glow again.
Now It's entirely possible that they figure out how to do some of these things without the help of computers, but most studios use CG since it's easier.