As the owner of a Wm8m with 40 couple hours just this year behind a Massey 1740m, I'll offer my 2 cents.
Bigger is 110% better. It's amazing how fast the 8" opening fills up when branches start folding in on themselves. If someone made a 12" opening pto
chipper that wasn't 2.5k+ lbs, I'd sell my 8m and buy it just to save on trimming. Anything over 4" is firewood in my books.
The horizontal chute of the
woodmaxx lineup is far more ergonomic than the upward angled chute of woodland mills/wallenstein units in my opinion. Why fight gravity?
My suggestion is either go for the wm8m to get into an 8" unit as cheaply as possible or get the mx9900 if you can swing it to have 9" and the hydraulic feed. I've only wished for hydraulic feed to reverse a jammed limb two or 3 times, so the forward only mechanical driven feed hasn't been a giant issue for me. I found my Wm8m used with 1.0 hours on it for $1400 so i wasn't about to argue with that deal.
Vermeer did make a 9" pto
chipper a few years back, they're rare but you might get lucky and find a used one. Not sure if they're as monstrous as the wallenstein 10" unit though.