Powerport+ 6 has 1 port which is QC3 and the other 5 ports have a 2.A max 5V. They combined must share 60W.
Powerport Speed 5 has 2 ports which are QC3 and the other 3 ports 2.4A max 5V, combined must share 63W.
If you focus on for example weight or size you see the 6 is quite a bit heavier and bigger, despite a lower wattage.
So its a decision of how many ports you want and if you want 1 or 2 QC3 ports, vs size/weight (not matters when at home but more so if backpacking/flying).
If you're wondering why such a difference in size given very similar total Wattage, these are different vintages.
As PowerIQ 2 works its way through the product line, expect it to physically shrink the 5 and 6 ports, oh and QC4 is around the corner, so depending on who shouts loudest you'd expect a refresh of one of these earlier than the other.
Given the current "dumping" of the Powerport 5 in a no-code price reduction, I'm expecting that to be a stock shifting move and newer Powerport in the next 2-3 months, exactly what exactly when I have no clue.