4 seasons,
Alot of this will really boil down to what kind of wood you have access to, and what size finished splits you need to produce. Pushing wood through a 6-way wedge takes quite a bit of power and puts a ton of pressure on the splitter. If you get lots of knotty or twisted grain wood I don't think you'll be happy with pushing that though a 6-way, you certainly won't get the speed you're looking for and you might just twist some steel.
As far as 6-ways go, I think you could go with a Built-Rite 11HPWS or 24HPWS (which I have) or Timberwolf TW-5, TW-6 or TW-7. Either BR can run a 6 or 8 way wedge, or a 10-way for that matter. I think the TW-5 and TW-6 can only run up to a 6-way, and the TW-7 has a box wedge. I also think split-rite has a 6-way. The cost difference between the units spec'd out with different wedges isn't really that much as the splitting wedges are usually interchangeable.
I also have a splitter rigged up to my backhoe, which is interesting. I built it up to break up "beast" rounds, but even with smaller rounds it's certainly not faster than running splitter from the ground.
I'm in NW NJ, if you're close and want to see my gear let me know.
-Mark