Each of the systems mentioned have different advantages and limitations, but you would probably be happy with any one of them as long as you understood it's limitations and accepted them.
The Dynamis is low cost to begin with with a street price of around $150; however, you can only use one throttle with the base unit. To be able to use any additional throttles, you have to get the Dynmais Pro upgrade, with a street price of $250-$300. After the Dynamis Pro upgrade, you are still limited to 4 throttles, which hardly seems like a "Pro" system to me.
The MRC systems are nice, but I do not like the fact that you can not use JMRI with it. They have a computer interface and proprietary software, but I plan to do integrated signaling and possibly even some automated running, which is possible with JMRI but not with MRC's software. as has been mentioned, the Express lacks decoder readback, and it also has a bug in the consisting function where it will not automatically setup an advanced consist with engines facing different ways, although you can still set them up by directly programming CV19. The Express also will not control accessory decoders. The Advanced 2 does cost quite a bit more than the Zephyr or Power Cab, but you do get true walkaround ability. With the Zephyr you would have to get an additional throttle and with the Power Cab you would have to get an additional throttle or a Smart Booster for true walkaround ability. By true walkaround, I mean that, with additional plug ins, you can unplug the throttle and walk to another location and plug in while your trains keep running. The Power Cab in it's original setup is a walkaround throttle, but if you unplug it the layout shuts down. If you compare the Advanced 2 to the Zephyr or Power Cab with true walkaround capability, they are in the same price range.
Some people do seem to think Digitrax is hard to understand, but honestly, I just don't see it. Some of their older throttles(which haven't been available for several years now) were not very intuitive because they tried to do too many things with too few buttons, which resulted in some odd button combinations to get things done, but I think that most people that have used a Zephyr and/or DT400 or DT402 throttle find them quite easy. Unless you just happen to like the "power pack" design of the stationary Zephyr throttle, you would definitely want to get a walkaround throttle for it. Personally, I really like the dual throttle configuration of Digitrax's DT throttles.
The Power Cab is the least expensive, full function, walkaround throttle, DCC system there is(as far as I know). What I don't like about the Power Cab is out of the box you can only add one additional throttle. For a small one or two operator layout, I feel the Power Cab is just about ideal, but it is pretty easy to outgrow. You can upgrade it with a Smart Booster, but that only expands it to four throttles. If you then outgrow the Smart Booster, your only option is the full blown Power House Pro, and if you have to upgrade to it, the Smart Booster becomes useless.
Something to consider concerning the two or four throttle limit for the Power Cab, especially if you might ever want to do PC based dispatching or control panels tied into the DCC system, is the PC interface and the input modules take throttle addresses, so they cut into the number of throttles you can have. Digitrax does not use throttle addresses so these items do not cut into the number of throttles you can have.