I tried mixing the two but it's tricky - if you don't get the timing and the dosage exactly correct (assuming there is a correct dose that combines well - I havn't found it yet) all that happens is the 5-apb takes away all the fun of the 6-apb and leaves you laid on the couch without the energy to stand.
6-apb by itself is a safe bet however.
I found 100mg 6-APB + 50mg 5-APB taken at the same time to be a wonderful combination, probably the most euphoric drug/drug combination I've ever taken. It far far exceeded either substance alone, and it was a lot stronger and longer lasting than I was expecting. Taking both together seems to only bring positive things to the combination for me.
I wouldn't dose the 5-APB any higher than that though, I think when the 5-APB dose begins to approach the strength of the 6-APB dose you'll be too floored to get up and dance and enjoy yourself quite as much (unless of course, that's what you're looking for, in which case go for it).
As for comparing the two, the ic50 values for 5-APDB and 6-APDB show that 5- is more serotonergic and less dopaminergic, and 6- is the more dopaminergic of the two. We can make a reasonable assumption that this carries over into 5-APB and 6-APB, and experience seems to reflect this, which 6-APB being the more purely stimulating and "rushy" substance, while 5-APB often leaving the user floored with not quite as much energy as they'd like. At higher doses the barriers between the two substances blur significantly and the differences are much smaller. 6-APB is also the more psychedelic of the two, and longer lasting.
As for comparing the comedowns, something a lot of people like to bear in mind, it seems to be an individual thing. For the most part I've noticed a trend that many people seem to get really bad comedowns from the more serotonergic substances, e.g. MDMA, 5-APB and the like but suffer little/no comedown from the more dopaminergic ones like Methylone, 6-APB, Amphetamine etc, and some get the opposite.
I personally always had the opposite, MDMA and 5-APB have little/no noticeable comedown other than a few moments of a faint nostalgia for the peak, nothing depressing or bad though - while 6-APB for example can get a little rough on the comedown from higher doses for me.