The advantages of a 
chipper with hydraulic in-feed.  You can chip twisted, angular limbs & trunks without having to cut them into smaller pieces.  The hydraulically powered in-feed roller will pull the material into the 
chipper.  Also - you can control the rate of in-feed.   There is an advantage to feed slower if you are chipping very hard, dry material or max size material.
If you will be chipping straight, soft material - pine, birch, alder, cedar, etc, etc - it may not be necessary to have this hydraulic in-feed system.
I have never found a need for anything beyond mechanical feed.  All the pines I chip are young, straight as an arrow and soft.
There is an advantage to a larger 
chipper - larger flywheel, higher air flow, larger input and output openings.  This has a limit - generally, don't go beyond the recommendations of the 
chipper manufacturer regarding required Hp.
Another consideration.  Larger chippers can handle larger material.  That has a limit also.  I chip my pines "whole".  Butt first - no limbs removed.   I chose the BX62S because it was larger but it has a 6" limit.
That is MY LIMIT also.  A green pine - 25 feet long -6" on the butt is all I can manhandle.  When I finish thinning a stand of young pines - it looks like a gigantic game of Pick-Up-Sticks.  There can easily be 120 to 150 cut trees lying all over each other and lying in all directions. 
So...... I grab a pine at the butt end and try to weave my way thru this pile without falling.  I drag this pine to a growing pile from which I will eventually back the 
chipper up to and chip.
Dragging to a pile is the PITS.  Actual chipping is fun.