What you get for the most part is better performance, More features or better materials.
if you use the MK2-R295 as a benchmark,
1st stage
This is a unbalanced, 1st stage and simple downstream 2nd stage. 1 HP and 4 LP. This is as simple as you could get right now.
from there you could jump to the MK11 1st stage and get 2HP/4LP port, and go to balanced diaphragm setup, the air delivery will be much more consistent, and if weight is a concern you could get this model in Titanium (MK11T) there is a Environmentally sealed version of this 1st stage as well the MK17.
The MK25 could be an upgrade on the MK11/17, as it offers a Turret, 2HP and 6LP, this is a balanced piston design that offers as much air as you would ever want or need. Hose routing can be easier on this 1st stage Vs. the MK2/MK11/MK17 (this is also available in Titanium MK25T)
2nd Stages
the R295 is a Very simple downstream regulator, there is no adjustment on it, put it in your mouth and breath.
From here you could upgrade to the R395 and that gives you the "Dive/Pre-Dive" switch while still a simple downstream regulator.
The C200/C300 are upgrades from here, you get a smaller 2nd stage, still a downstream but adjustable with an air barrel. this means it easier to tune the air delivery as needed.
From here you have the G250V a larger 2nd but great for ease of breathing and cold water reliability has a dial to adjust the breathing effort on the fly, great for diving in a current.
next from here is the A700/S600, these are completely balanced 2nd stages that are completely adjustable, the G250V is balanced though its adjustability is reliant on the tensile strength of the spring in the regulator, on the A700/S600 this spring can be preloaded with an adjustment screw.
Hope this helps.