The DR 670 is also good as far as reviews are concerned and nice to know more from you. Geoshred yes I knew about it and then forgot about it. I would like to use it again but since I have 6 guitars right now iO suppose it will stick with me for a long while. The screen is good for playing like a Hawaiian guitar and it’s kinda nice for electric solos and shredding, and possibly as a sound source for sampling as well (phrases, riffs, power chords), but I see it as a shortcut without getting finger burn 
Might be great for a lot of other styles though.
Regarding SP404SX you are right it has many features lacking but for what it is if I can get it at a bargain price this toy will fit somewhere right in my kit list, I can see a certain use for it. It does not have midi out and pads are fewer and not velocity responsive and sample editing is crude with a mostly numeric display. Akai MPC 500 seems to be the right competition that defeats it easily at this price point and feature set comparison. However it seems a lot of hip hop artists use it or have used it quite well since it came out in 2005, and end of the day it can seem to get some decent results if worked on well through and through and if the sampling is really well done, as those producers have spent their time honing their craft to do well on any gear whatsoever. It’s also a discontinued product so it’s just fine for having fun in order to spend some extra cash. An MC 505 seems the best bet for now it seems considering everything else, even if it does not support sampling.