I thought I posted on here a couple days ago from my phone, but I guess it disappeared!
Some random thoughts on the subject:
I bought a big grapple truck, 76 yard bed, 500hp, 18 speed, tandem with a tag axle, rear mount 27' Rotobec 80 loader with a 76" Rotobec debris grapple. I'm also playing with the idea of building grapple trucks through Compact Equipment Store. It seems like a logical progression considering I usually try to sell what I use to do tree work.
Most grapple trucks are going to end up being roughly the same height as most loaders are going to end up being around 13' tall. I wanted to build the truck under 12'6" tall, but that's not terribly practical so I figured the bed height might as well be as tall as the loader (or a pinch higher to protect the loader).
If I was wanting a "medium" grapple truck, I would still want a tandem. I'm also a big fan of rear mounted loaders. I was on the fence HP/Transmission wise on my truck, but now that I have it, I would hate to be without. The truck will need a double frame or a hd single frame to take the stress of the loader. I prefer to have a smooth exterior for easier signage and the like. I've yet to sign mine, I'm repainting the bed and work has got in the way to finish that.
Loader wise, I'm planning on getting to a Palfinger Epsilon M13A98 which has ~32' of reach and can outfit Rotobec handily. Aside from the reach (and shorter retracted length with it's two stage arm), the protected hose routing is also alluring to me.
Even on a medium truck, I still think I would have a tag axle. A tag is behind the drives and a pusher is in front. With a tag you can move the drives farther forward, shortening the wheel base and making the rig more maneuverable. Also, you can still tow a heavy trailer behind the truck without overloading the drives or making the steer uncomfortably light.
I have a 30' pintle trailer with a hydraulic dovetail on order which will be here in a couple weeks. (I have a dealer license now for everything but new light vehicles). That trailer is going behind my grapple truck to haul two machines behind the truck so I'll have a one truck wrecking crew for most of our jobs. I can haul the lift with the excavator or track loader, or the excavator and track loader, as well as a mountain of debris in the truck and two people in the cab.
A medium sized truck could save a fair amount of empty weight, giving you more payload than a larger truck. With a rear mount you also have the option to tow a trailer to load debris in as well, although given my truck's size, that proved to be a little much to be practical for most jobs (155 yards combined) considering the empty weight.