I have no experience with the MC 808 or 909, but one of the first thing I started making beats with was the MC 505, which I liked quite a bit and it taught me A LOT about sound design and production in general. Most people, it seemed to me, used it for techno or other electronic music production, but I used it for underground, boom-bap style hiphop beats. I later incorporated a sampler (SP 202then later SP 808, which I still have and love) and that certainly helped for more of the acoustic drum stuff/layering and other sample type production.
It is important to know that these are not synths with their own sound engine. They have a databank of sounds (samples) that you can edit/alter, AKA Rompler. The MC 808, as I understand, needed to be connected to a computer in order to alter patches, kinda dumb because if that's the case, then I'd just go software anyway, just like the case with the MPC renaissance. The whole purpose of the MPC and the grooveboxes was to have a stand alone unit that did not rely on computers. There was/is a sort of devolution there for lack of vision with these companies (my personal opinion).
It allows a lot of editing possibilities and IMO is way better than some of the modern options, for the price, but I have also not checked out the MV8800. I've seen it but not played with it at all. I've seen MC 505s going for around $200 and for what it does, its a fuckin steal if the unit is fully functional. It pisses all over these most recent groovebox renditions IMO and it's great to be able to have a more hands-on approach to making music, especially when you are starting out. I think the brain grasps more information this way (especially for males or visual learners), and it's really a lot easier to understand what's happening sonically/physically if you have a tangible relationship with an instrument vs software production.
I still have mine, I've had it for 15 years. I'm going to use it to teach my son how to make beats.
I look at the interface of both the MC 808 and 909 and still think that the MC 505 is a better machine with more immediate access to tangible control over the sounds, but it does
not have the ability to sample. Of course, I am biased, as is almost anyone who has invested time and money into a piece of equipment (you will notice that almost all people online will try to convince you that whatever they bought themselves is the best investment.) We might all be guilty of this