Since I built my first e-bike over 3 years ago, its been 12s LIPO all the way for me . I also hobby with electric RC helicopters/multicopters and RC planes so using my stash of LIPOs and infrastructure of chargers, connectors, tools, and wiring for e-bikes was a natural extension of my electric RC hobby. I use the eBay special 1000W direct-drive rear wheel kit that gives plenty of torque and hits a eye tearing top speed of 28mph.
I use pairs of 6s cells for 12s (50.4V) of power. The Anderson Powerpoles ability to create series connections on-the-fly is like magic. My bikes have a parallel "Y" cable with 6 leads so I can hook up as much or as little power as I need for my trip. I've easily biked around with a tiny 12s3000 for neighborhood store runs but have room for the equivalent 12s45000mAh (or ~2160Wh).
My 1000W brushless motor pulls at most 20A. Getting 20A of current draw actually requires full throttle and going up hill a bit. Still, this "small" 20A of current is paltry for the 25C+ discharge capability LIPOs (versus 2C for the 18650 Lithium ion). In fact, the LIPOs barely get warm and give gobs of power all the way through to the end. Hill climbing power with my rear wheel direct drive motors is awe inspiring -- that's when LIPOs really pull through for me as direct drive causes the highest current draw versus geared motors. But there's no hill in San Francisco I couldn't get up on. I've read reviews praising mid-drive geared e-bikes versus direct motor drives saying how much better they are for San Francisco hills -- I can tell they weren't comparing against a pure LIPO powered direct drive bike.
The 1000W rear motor on my bike may grind away on a steep hill but the power keeps pouring out of the LIPO packs non-stop!
I recently bought four of the Multistar 10C 6s10000 LIPO packs. These are now available in the HobbyKing USA warehouse:
https://www.hobbyking.com/hobbyking/sto ... ouse_.html
At $98.11 for 6s10000mah, it works out to $0.39/Wh. Of course, you need a decent quality balance charger. I have a 2000W $360 LIPO balance iCharger 4010Duo along with my old trusty 1000W FMA PL8 charger but these chargers also get used for other stuff particularly my RC hobby.
These Multistar 6s10000 packs are only rated to 10C output -- which, on RC terms, is small. However their 10Ah size makes up for somewhat but still overkill for the 20A requirement.
That all said, LIPOs are kinda overkill for a 20A e-bike application. As an experiment, I've purchased a set of UltraFire 7200mAh 26650 Lithium ion cells. I'm going to build me a pair of 6s1p 7200mAh flat packs (with balance leads) and do some test rides with an equivalent 12s7200 of capacity when two of these packs are put in series. If they work to my satisfaction, I may build more of them. I want to have at least 12s14400 as these Lithium-ion packs will be working harder with their puny 2C output. Their output rating is small but they are half the weight of LIPOs for the same capacity so its worth one experiment at least. I might be able to do a "hybrid" approach and have a mix of 6s LIPO and 6s Lithium-ion in parallel. Gotta be careful of pack equalization as the Lithium ions only do 1C charge rate versus the typical 5C LIPO charge rate.