With the Ruixu, for an extra $1500ish with 6 you get (eggroll?) a pretty nice rack, a fat bus-bar and cabling to it. The batteries appear to be slightly lower profile as well. Probably 1/2 the difference if you want to rack it all up with a DIY arrangement. I'm just a little too anal, Thus far the RUiXU rack has been the most impressive cosmetically that I've seen in a rolling rack arrangement.
That being said, I'd buy $1K 5000kwh pre-assembled rack mountable batteries before I' d try and roll my own. At that price point, it gets increasingly more difficult to justify the diminishing savings difference from building it yourself.