How much difference would I see between a 26" and a 28" using 150 grains of Pyrodex?

The projectile might be a few fps faster, but accuracy and every thing would be about the same. A lot of inlines are using 24, 26, and 28 inch barrels and they all shoot about the same. Actually even a 22 inch barrel does a great job. You just don't push magnum charges through them. Also you will see that magnum charges are not really needed.

my guess not much. 150grs is not really needed. I shoot from 70grs to 90grs due to the fact I am a little bit of an recoil wimp (bad right shoulder). also I run test that tells me the max. of powder each of my guns can burn.

Thanks I am looking at the dream season compared to the bone collector and was concerned about the shorter barrel on the dream season.

So far as velocity goes, with normal "most accurate" hunting loads the additional two inches would be neglible. Pick the one that fits you best.

You'll notice more of a difference shooting a Non Magnum Load than the barrel length. Accuracy is Key, seeing the Animal is next and getting the shot where it counts is what matters most.
I do prefer a Longer Barrel only because Im 6'6" and I need a longer Mler, and it does add a bit of distance to your shot by burning just a tad more powder. It also helps to Stabalize the Bullet a little more too, but when you'r only talking a 1" or 2, then the effects are minimal.
(BP)

Barrel length does not affect accuracy. If you use iron sights the length of the sighting radius has an effect on accuracy.
If you use light loads the difference in FPS would be negligible the heavier loads will show a bigger difference. With magnum load and black powder it could make a noticeable difference.
Pyrodex is a relatively slow burning powder so with a heavy load and a light bullet the difference will be noticeable at about 100 gr.
If you only use the gun for 100 yds and under you will not see much difference once you start shooting 200 yds it will show up.

In order to burn 150 grains you need a gun with a barrel like this!

This a goose gun and that is a standard door height next to the gun. Seen this on the net a while back and looks like fun to shoot.