Don't buy a manual feed.
I started with a 4-5" manual feed small trailer mount with
a 16hp engine. The vibration on your hands/arms is incredible.
I'd use it for twenty minutes and my hand and arm would be
numb. My wife watched one time for about 10 minutes and
insisted I get rid of it and buy a hydraulic feed. With only
8 hours on it, I sold it for a $1000 loss (1/3) and bought
a 25hp 7" Gravely hydraulic feed. Still a trailer mount but that was
before I had a tractor. Yes, the hydros are 2-3x the price but
your arm is worth something over the next few years. Research
high-vibration induced medical effects like whitefinger disease.
A 6" is really only good to about 4" without stopping/starting the
feed to keep the engine from bogging/stalling.
I'd debated last year whether to sell the Gravely and
replace it with a PTO hydro feed (likely Wallenstein) to eliminate
yet another engine but decided I'd about even out $$ wise.
Other reason is that chippers (maybe humans?) run for 45 minutes
to an hour straight during chipping sessions and over the long haul, I'd
rather replace a 25hp Kohler than my tractor's diesel.
If I had to start from scratch, I'd buy the Wallenstein. I think
it may come with only one feed roller but could be increased to
two? If so, get two. I started with one and two (upper/lower
vs. just an upper) eliminate most cases of the branch slipping.
We run all hardwood oak so softwoods may be a different story.
I'm like you, anything over 3-4" (depending on my mood)
goes for firewood. Still leaves a lot of branches. You'll
find hydraulic feed speeds up things doubly. First, it can
stick 'er in with more force than you can so that part is
quicker, but most importantly, you start a feed and then
walk to the pile to struggle to ready the next candidate
while the previous is being ingested. That part is HUGE!
Pulling the darn sticks out takes the most time and certainly
is the most frustrating part. They fight back!
When the Gravely dealer dropped ours off as a demo in 1998
I had a test pile of branches that would have taken me an
hour with the manual feed. Granted, the machines weren't
the same capacity but the 7" took about 9 minutes even
when it was still equipped with a single infeed roller.
Sorry for the long post, just a BTDT response to save you
the trouble.
My $0.03 -- Mark