This one is impossible to say, it depends on your specific needs. For what I do, I find it convenient to run packs in 2p then series multiple 48v 10 ah packs as needed for the range I need that ride.
So for example, with usually 4-6 ebikes in the garage, one needs the lightest possible battery, one 48v 10 ah does er. Another might want 100 mile range, that one carries multiple 10 ah packs, plus other chemistry packs I have. My needs are served by having multiple 2 p packs. I made a bunch of y adapters, that make two packs into a 2p pack, and those have been used on one set of packs and another, for years now.
I like the idea of a smaller fire inside the house, and 48v 10 ah fits in my ammo cans, which fit in my flat shovel, to heave out the door in an emergency. Maybe that will work,, depends on how bad it is when I run in there. The idea is that if I do get a fire, its a 500 watt hour size fire, not a 2000 watt hour size pack going off. Either one would burn the house down easy, but my fireplace can contain a fire in the 500 watt hour size. Not so sure it could contain a fireball 4x bigger.
Others like Wes,, find series first then parallel is much easier, and results in much less tangle of wires. This would make sense for me too, if I had one bike, and wanted one big battery. This way, you need less of those home made Y connectors. I use a bunch of em, so my bundle of wire is bigger.
What you do with the balance wires gets even more individual, I like to say 1000 ways to do it, all right, one way wrong which you know right away because the wires melt in your hand. That way often happens when you switch from say 18s to run, then to parallel all in 6s to charge. Disconnect the 18s connections first, THEN connect balance wires in parallel.
Personally, I never connect the balance wires parallel anymore,, hey, that solved that one easy.. I just run in a way that results in needing to balance only about 3-4 times a year. So the hassle of balancing each individual pack is not really a hassle at all , if its that seldom.
