There are a few important factors when making the choice between the two machines.

1. How heavy of a tool can you work with? At 19lbs the 6 3/4" is a big tool to be using all day. The 12 1/4 is 40lbs. It does however travel fairly well on the timber with the roller in the front.

2. How much are you willing to spend? Last time I checked you could get 3 6 3/4" planers for the price of a 12 1/4"

3. How much time will you spend straightening timber? The 6 3/4 works great for making your winding sticks parallel. It can be done with the 12 1/4 but its more work. I'm currently contemplating a jig that would make short work of straightening timber with a 12 1/4.

4. How much time do you want to spend on the end of a planer. The 12 1/4 can do in one pass what takes 2-4 passes with the 6 3/4. You also get a crisper finish. The 6 3/4 gives you a bit of a rounded profile if your not careful.

I recently bought a used 12 1/4 planer after working on a frame one summer with a 6 3/4. I chose a 12 1/4 planer with a boring machine, when I could have bought a mortising machine and a 6 3/4 planer. I often work with a carpenter that has both the mortising machine and the 6 3/4 planer so the boring machine and the 12 1/4 planer bring more variety to the table when we work together.
In time I hope to get the mortising machine, the 6 3/4 inch planer and a large circular saw. But not for my next project.