With all the differrent apps, and such different naming conventions for each, it's not easy to figure out a hierarchy of complexity or ease of use or anything like that.
e.g. with other terms in their names, this one in this present thread seems like it's the 'granddaddy' of them all, but perhaps not.
Up till now, I've always set my thermostat by time, and that's how I started configuring it in another tstat SmartApp I'm testing. However, I think I'm leaning toward using the Modes method instead.
I think I like the idea of extending my knowledge and use of CoRE by including this in my rulesets. However, since I'm still a CoRE n00b, and since there's an issue with CoRE, AND since a friend of mine just got a smart thermostat too, and needs some assistance getting going with scheduling, etc, I figure I might as well use a SmartApp for now, and have the added benefit of being able to introduce him to something more purpose-built and ready to go. I like that, by being a purpose-built SmartApp, there's a certain amount of guidance that happens just by the structure of the app, the order of questions, etc.
I guess I won't really know for sure which route I want to go (that is, SmartApp or rules) until I give them both a try.
So, I will give your 'Ultimate' mode change one a try.