I just planted 300 hybrid poplar this spring, so feel like I should jump into this conversation. I used a middle buster last fall to bust furrows about 12 feet apart running "on contour" across my sloping hay field. Turns out the furrows do a great job of catching rainfall runoff (cause they're on contour) which benefits the trees that I planted in them this Spring. Middle busting also threw the sod back out of the way far enough that weed competition with the small trees was greatly diminished (although I do take a trip through with the weed whacker and touch things up every month or so).
If your planting trees with root balls (as I was) you'll still have to go tree by tree and dig a hole for each one (I did this all by hand and it took me and a buddy 10 hours to get 300 trees in. My knees ache just remembering it). If you're planting a tree that grows from a cutting (like hybrid poplar or hybrid willow) then you could just make a slit deep enough to accept the cutting and move on.
I'm not sure the subsoiler would work out so well. I can see where it would give you a nice deep slit to work with, and if you're putting in cuttings that's a good thing. But you would still have to contend with competition from weeds, or dig a hole if you're planting anything with a root ball. One benefit of the subsoiler is that if done on the contour it would capture rain fall runoff just like the middle buster furrow.
After learning from this year's experience, I'll be trying another approach next year (I have about 1000 more hybrid poplars to get in the ground). I planned to buy a tiller for my tractor this winter. In the spring I'll use the tiller and make passes 12 feet apart, on contour. This will kill the sod and minimize weed competition, and allow me to plant cuttings vice root balls (giving my knees a break).
So what are you planting?
KVMAPR