Personally, for the limited use you're going to have, I'd use which ever one I could find for the lowest price. With the limited use you have, you won't notice enough of a difference between a 461 and a 462 to actually tell them apart. If you do notice a difference, it likely has more to do with how the saws are tuned than the actual capability of the saw itself.
To me, the 400 replaces the MS360 family of saws. At only 1/2 lb more weight, they produce considerably more power and make a great middle saw for a 3 saw plan. You can run a longer bar on them occasionally, but similar to the 360 family, they aren't quite big enough to be a big saw in most 2 or 3 saw plans. If you want to run a 28" bar occasionally, but plan on normally running a 20"-25" bar, I think the 400 would be a great fit. It would make a great work horse for most of your bucking needs. If you're looking for something that will pull a 28" bar on a regular basis, or you think you might need something longer on occasion, my opinion is that you're into the 460 family of saws. If you're needing to run a premium bar oil to support the bar length, then you're bar is too long for your primary firewood saw. Even if you're normally running a 24" bar, there are things you can do with your chain that will let you take advantage of the additional HP available compared to the 400.
As far as standard carbs vs Mtronic, I'm firmly in the standard carb category, but I don't actually think there's any reason not to buy an mtronic. They've been on the market long enough now that they are proven. The reason I don't use or want them is simply because I want to be the one who decides how my tools operate. I've spent the majority of the last 3 decades working on automated equipment in hot, humid environments, and I've seen what the heat does to electronics over the course of decades. Mechanical wear, stiff or cracked rubber, corrosion, these are things I can see and trouble shoot. A faulty chip on a circuit board, however, can be a nightmare to diagnose. In a saw, however, the answer is normally to simply replace the entire component. So long as that component is reasonably priced, I'm fine with it. Every Stihl dealer I've ever stepped foot into was WAY more interested in selling me a new saw than they were in keeping an older saw running. If they tell me I need a new saw because I've been running the same $5 sparkplug for 6 years, I don't trust the to tell me I need a $20 solenoid instead of a $1500 saw. Basically, I'm of the mind set that I want to know enough about my tools and equipment to be able to efficiently maintain, operate, troubleshoot, and service them. I think a person could do that with Mtronic saws just as easily as they could with a standard carb, but I'm also pretty stubborn so there is that....