One of my first "big" purchases was a man lift, which was nice, but it wasn't a game changer. It allowed me to do aerial work more easily, could do some light duty craning with it's 500lb capacity, and return to the ground whenever... but it didn't make a huge difference. The chipper was much the same way, it helped, but it wasn't huge.
The first huge thing was the mini. We had the lift, but now we could handle tree debris without sweating! It didn't matter how big the trees were, we could move them much easier, faster, and farther (albeit with some cutting!).
The next big thing was a tandem single axle, tag dump trailer I got from Arborworks... simple thing really. Now we could load with the mini and dump at any site without the mini or sweat. Also we could haul dirt, gravel, logs, chips, brush, concrete etc.
The next big thing was upgrading to a tandem dual gooseneck dump Skwerl turned me on to and clean diesel F550 I found in Ohio. The gooseneck offers far more maneuverability, and the F550 was easily double the truck of the Chevy 1 ton gasser I had been using. Hell the replacement engine and the rebuilt transmission in it cost less than the F550!
Then we had the Gehl AL340 then AL540. They could load over the side of the dump trailer, could dig, handle full size logs, travel ~4x faster than the mini skid all while not damaging turf. Basically a ideal forwarder for doing tree work.
From there I've bought a 100hp track loader, cab/air 13klb excavator... I hope to buy a cab/air AL540 sometime this year. The track loader is a bit much and expensive to run on some jobs. Also the visibility of the AL540 is far above and beyond the track loader. However the track loader can just about out push 2 AL540's, but it burns 3-7x more fuel on tracks that cost over $5k. In fact, list price on replaceable parts of the track loader's undercarriage is ~$24k! But now I can whoop ass at tree work, small scale dirt work, clearing, demolition, etc etc.
The hope is I stay busy enough with my diversticity (new word!) that it's a positive gain by far. 2013 worked out that way, hopefully 2014 will be on par or better! My area is fairly low in population, I don't believe it's practical to stay busy doing tree related work alone.