So short answer is that the Kmpkt Dynamo is the best and you should just wait for it and forsake all other DC-DC boards. The real answer is that they all vary in several respects. Prepare yourself for walloftext!
The first major difference between these units is the form factor. Some units are a Pico-PSU style build (HDPlex 160W DC-ATX is the best current example) while others are a fully modular flat PCB like the HDPlex 300W DC-ATX. The Pico-PSU style units are relatively limited in terms of the wattage they are rated to handle. Presently the beefiest units in this form factor are rated for around 160W. That being said, a lot of people run them around and even slightly above 200W and report no issues. If you want to go over 200W with your system, then you're going to need a modular PCB based unit like the HDPlex 300W DC-ATX. The upside with these is that they can push a much higher amount of power, with the obvious downside being that they are significantly larger and require significantly more cabling due to the almost universal need for a 24 pin ATX cable.
The second major difference is input voltage. The typical input voltages are considered to be 12V and 19V. 12V units are preferential from an efficiency perspective as given the 12V input is the same voltage that is meant to be supplied to the motherboard, very little conversion of voltage needs to happen onboard. This means less heat to dissipate and a cooler system. The downside of 12V units is that the brick selection is generally quite poor with quality generally being poorer and cost higher than their 19V counterparts. Additionally AC-DC units converting to 12V are a couple of percent less efficient than their 19V brethren meaning a hotter brick (theoretically). 19V is vastly more common and you will have a much broader selection of AC-DC units to choose from when using this input voltage. The main reason for this is that laptops use 19V bricks, so there has been a lot more time and money poured into creating better 19V AC-DC units. While the conversion of wall current to 19V is more efficient than with 12V (i.e. theoretically less heat at the brick), more conversion has to happen at the DC-DC unit which means more heat to dissipate inside your case. Overall from wall socket to motherboard, 19V systems tend to be slightly less efficient overall.
The third major difference is going to be quality. There are a lot of different products on the market right now in this segment and they vary wildly in how reliable and well-built they are. HDPlex seems to be the most commonly used brand around here from what I can tell and their quality seems to be great. Furthermore their customer support is awesome (which is part of why I chose to work with them on my own DC-DC solution). G-Unique also looks to be a really good outfit and so far their involvement in the community here has been very impressive. They also have representation on this side of the Pacific (much like HDPlex) which should make customer service and communication much better than some other brands. The old standby is Mini-box and their Pico PSU XT-160. This is a really good unit and runs really well off the Adapter 192W brick. They are kind of a one trick pony however as they offer no other compelling products for our community. We have had a number of people get units off of Aliexpress, myself included and I do not believe the experiences were particularly good. I bought a unit called the Z4-ATX-200 from a distributor called Pico Box and it lasted all of a week having never pulled more than 130W from the wall (I run a Kill-A-Watt to my PC regularly). When I contacted them for warranty, they told me I had to ship it back to China and would only get another unit if it was inspected and found to be faulty. While this isn't completely unreasonable, the cost to ship back etc. was more trouble and cost than the unit was worth. I am not the only one who had problems with this unit.
Finally to clear up what I think is a bit of a misconception you have about the AC-DC end of the equation. While the bulk of the AC-DC power supplies available are in fact external, there are a couple of viable internal units now on the market. The easiest to obtain is the HDPlex 160W AC-DC and the soon to be released 300W version of the same unit. If you're comfortable with electronics, Meanwell also makes two lines of internal open frame AC-DC units under the EPP and RPS series names. The advantage of these is that they are easier to cool being open frame and they also go up to 400W which means they presently offer the best power density of any internal AC-DC solution.
I hope this answered your question adequately.
