The tow behind boom lifts are great, I have used them up to 50 ft.
The great thing about them is they self level, unlike the larger and heavier 4x4 self propelled booms. So you can use them on serious slopes.
You hold one button and all four stabilizers come down until the thing is perfectly level. I've had the big machines lock out and refuse to operate on ground that appeared level, had to shim the tires to get it level.
For just one limb I would probably just use a 40 foot ladder and a hand saw,
You do have to tie yourself off to the tree, as the falling limb can hit the ladder. You should have a second person who could pull the branch away with a rope. It's a little hairy working from a ladder in a tree. I'm used to ladders and it's still a little tricky, so if you didn't want to go the ladder route I would support your decision.
If you use the boom lift, come up from behind the trunk and get as high as you can while still being able to reach the limb. That will put you in the safest position.
Ideally you would use the boom for as much work as you can find for it that day, but they are reasonably priced enough for just one task I guess.
I do what Aaron Z mentioned with the 3 pt trailer hauler, works perfect as it floats up with the machine as it levels, plus is much easier to maneuver into position. The trailer tongues on those poor things are usually beat to all heck from the weekend warriors trying to get them into tough places in their backyards with their pick up trucks.
I try to line up a couple of paying jobs, then bring it to my house to trim trees. I even bring the tractor on the jobs, if I have to move it around someones yard.
These are all pics of the 50 ft JLG. One of these pics is at max reach on a steep incline. These are very well designed and built machines, but they are pretty light duty compared to the commercial lifts we're use to. so it's a little hairy on these little guys when maxed out.
They do have a little more play in them.
Good Luck!
JB