I've got a 5 ton, 8 ton, and a 15 ton. Out of those 3, our 8 ton is the least used, and least liked.
The 5 ton is small enough to get in to most places, but big enough to load a tandem or triaxle. It's a real gem.
The 15 ton is big enough to dig basements, run a real hammer, demo 2-3 story buildings, and load trucks productively.
The 8 ton on the other hand, isn't really any of those things. Needs a deckover tag to move it, which for us means it needs the same truck and trailer as the 15 ton...so we may as well take the 15 ton. I bought our 8 ton before the 15 ton, and if I had to do it all over again, I wouldn't buy an 8 ton. Aside from overhead clearance, the 8 ton and 15 ton have a nearly identical working footprint, as the 15 ton is a zero tail, and the 8 ton is conventional.
My 2 cents - I'd keep running what you've got, until you've got enough work to justify a full size, and make the leap. Lots of guys around here hire out all their moves. Truck and trailer maintenance/insurance/operating cost ain't cheap. We've got 10 pieces of iron to move, so it pencils out for us, but I still hire out moves if we're busy.