I have my scale packed away somewhere so I didn't weigh the 44.4V 5Ah LiPo pack (222Wh) however according to H-K it should be around 3.5 lbs. My 48V 10Ah 15S LiFePO4 (450WH) tips the scales if I recall correctly right around 10.5lbs which means LiPo is about one-third less weight to tote around.
I was torn between going with 12S, 13S, and 15S LiPo. The merits of 12S was simplicity with just 2 batteries which could be balanced with 2 HK battery medics. 13S was also in the running because it could use the same 54.6V charger I am using with my 15S LiFePO4 pack. 3x5S was also an attractive idea for speed but as you noted it would have been pushing my controller and WU meter too high I think. Also since I was only going with 5Ah the tempting higher speeds no doubt would have quickly drained the pack. When it came down to it though simplicity
won out. I was also considering running the LiPo pack in parallel with the LiFePO4 pack and while 13S might seem the choice there because of the same charging voltage in actuality 12S won out there too. For example running a no-load speed check with each freshly charged battery (about a 1A load) drags the 10Ah LiFePO4 battery to 51.0V and the 5Ah LiPo to 50.3V.
Yesterday I did parallel the 12S LiPo with the 15S LiFePO4 but not after first blowing up my WU meter when I totally flaked and wired them in series
Anyway I did then make up a proper parallel harness with a Schottky diode on the LiPo side. With the batteries in parallel I should have about 672Wh available (45Vx10Ah + 44.4Vx5Ah).
I ordered 2 Turnigy wattmeters from H-K but for this ride I had no way to know how much each pack was contributing. The only monitoring I employed this outing was the LED voltage readout on my light but that was enough to keep me from draining the batteries too much. I used the motor nearly 100% of the time and covered 45.4 miles at 18.1 mph which is further than I've ever traveled at that speed
(I've covered 65 mi on the LiFePO4 alone at 16mph, with a LOT of pedaling) . At the end of the trip I still hadn't seen the red LED illuminate on the headlight which it will do at a hair over 43V. From past experience I have found this point (at max current, ~20A) represents 70-80% DOD on the LiFePO4 battery and is a good place to stop. When I receive the new wattmeters I'll be able to get independent power consumption data from each battery.