Lemme see Kev,
Leaving aside higher power lasers (1Kw +)
RF = Radio Frequency tubes, they use the same/similar gas as a DC tube but the beam is generated by an RF signal
DC = Direct Current tubes, they are excited (the beam is generated) by a very high voltage arc inside the tube (circa 20,000 to 45,000 volts)
DC tubes (in general) do not have the availability to have their number of pulses per inch varied, RF tubes do.
Your ULS and Gravo are "likely" RF tubes.
Different materials have different values at which they absorb radiation (the beam) a CO2 laser provides a beam of 10,600nm (in rough terms) a YAG and a Fiber laser generate beams in the 1064nm wavelength.
Aluminium for example is highly reflective to the 10,600nm wavelength but absorbs 1064nm wavelength very well. Therefore aluminium will require far far higher power to cut/mark if you use a CO2 laser compared to low power for a Fiber or YAG.
A 20 watt YAG/Fiber will outperform a 200 watt CO2 on aluminium because of this.
Wood absorbs radiation (the beam) in the 10,600nm wavelength very well, but does not in the 1064nm wavelength so the CO2 laser will outperform a YAG or a Fiber on wood.
Wavelength:
This controls the smallest possible size of your laser spot/dot.
A YAG or Fiber can in theory produce a dot 10x smaller than a CO2 making them very good for fine detail work (depending on the material type etc) as well as being able to mark directly on most metals.
Chinese YAG / Fiber Galvo markers start at around $10k and go upwards. (be aware they are pretty poor if you want to cut items though)
A triumph machine with an RF source is like fitting a Ferrari engine to a Skoda, you can do it but do you want to? you would end up with a $5k machine with a $10k laser in it if you wanted 100 watt or so.
My apologies if this comes over as overly simplistic but I'm not sure quite where your level of knowledge is of Lasers in general.
cheers
Dave
ps:Z series tubes have better internal mirrors and slightly higher peak powers but past that in real terms not much different. To be honest I prefer EFR tubes as they seem to be more stable.