Many second stages have removable face plates, but you also want to keep the parts count low. The G250/G250V for example, has 1) the cover, 2) a friction ring, and 3) the diaphragm. Other second stages may have a 2 or 3 piece outer cover, and/or a screw in inner cover plus a friction ring and a diaphragm, and at that point you have an elevated risk of losing one while removing the cover under water.
I'm more inclined to remove a cover in a cave than I am in open water. In many caves there is little or no flow and the environment is fairly controlled, even in high flow caves, I can usually find a spot out of the flow, and if I have an issue with a deco reg, there is usually a hard bottom available to mess with the reg. However in open water, there is surge, current, and quite possibly the problem of having to work in mid water and worse, mid water in current while clipped to the up line with a reel or jon line. That reduces the chances of success quite a bit.
I'm not a big fan of the Mk 20/Mk 25 first stage. I used them prior to the Mk 17 but switched when it came out and have never regretted it. Scubapro still markets it as it's flagship first stage, especially in the use (less so in Europe) but it really isn't. The Mk 17 has better hose routing, is far better in cold water and is far less prone to contamination.
Similarly, while I have nothing against the A700, the G250V is a much better second stage for technical diving. Now that they've put a metal air barrel in the S600, it runs a closer second place choice. The trade offs are in 1) a smaller diaphragm that provides a bit less power for the same internals, but fits under your chin better for a back up second stage and 2) greater complexity and more things to fail - which isn't really want you want in a technical reg, especially your back up reg.
So I'd suggest a Mk 17 G250V for both primary and backup regs, or alternatively a C300 for the back up second stage. The C300 is simple, has good performance and is basically bullet proof.
-----
There's lot of Hog love out there, but the problem is you never quite know what you are going to get. Any reg available at that price point is going to suffer from the same problem. Tolerances and QA standards are directly related to cost. Looser tolerances and lower QA standards mean the OEM can sell it to the company for less money. Tighter tolerances and higher QA standards increase the cost to the company and to the consumer. But along with looser tolerances and lower QA standards, comes greater variability, so while a reg like the Mk 17 G250 is very consistent, meaning any Mk 17 G250 will perform like any other Mk 17 G250, the odds of that being the case with a large number of HOg regs is a lot less. You might get lucky and get one on the great side of average or you could get on the bad side of the average. And in terms of individual parts, the same thing applies - you run a higher risk of getting a part in the reg that was not properly finished, de-burred, plated, etc.
Consequently, below a certain price point, you really are getting what you pay for and based on that that, supported by what I've seen come across the repair bench, I avoid bargain priced regs in my personal diving.
