To me battery selection begins with the motor and controller selection. For example say you pick a "typical" kit.
It's a mid sized direct drive hubmotor with a 22 amp controller. So now you know you need a battery that can provide at least 22 amps in a burst, and say 15-18 amps continuously. Now you decided how far you intend to ride. Your use is likely to fall somewhere between 1 and 2 miles per amp hour. I like to use 1 amp hour per mile for full throttle riding. Usually you use less, but on windy days you might use more.
So say you need to ride 12 miles. You decide to cut it fine, and buy a 10 ah battery. So now where do you stand? you need a battery in 10 ah size that can put out 22 amps. A 10 ah battery at 1 c will provide 10 amps, so a 1 c rated battery is not good enough. A 2c rated 10 ah battery will provide 20 amps. You are getting close now, but a bit more might be better. So for a 10 ah size, you need to look for a battery type with a 3c discharge rating.
If you decide to buy a 15 ah battery, then you have 30 amps with a 2c rated cell. This is why I often recomend a 48v 15 ah battery to those that want to run a bike kit on less expensive prismatic lifepo4 that is rated 2c.
But say you only want to ride 5 miles. That's pretty short, so a higher c rate battery could be used allowing you to use a smaller pack. In that case, a 5c rated cell would give you a 5 ah pack that would put out 25 amps. This is the lure that leads some to choose high c rate lipo. They can do thier short ride with a very small and light battery.
So as you see the answer to which is best, changes depending on what your limitations are, cash, distance to ride, speed you wan to ride, etc. Personally, I just find it easier to buy a largish lifepo4 pack, and have lots of range than try to calculate my needs down to the wire. For me, this works since I have to have pretty long range anyway, living 15 miles from work. For the 400-800 watt bike kits a 36v 20 ah or 48v 15 ah lifepo4 is a pretty good choice. The battery is not too heavy or bulky( at least compared to slas) and carries ok on rear racks. But it's big enough to provide 30-40 amps for the hills, and go at least 20-25 miles. My impression is that even if you have a short ride you need to do, you will soon be enjoying longer rides if you have the capacity to do them. For me, 10 ah is just too limiting.
Another thing that affects the decision is cycle life. Lifep04 generally wins this contest, with 1000 or more cycles. Other types tend to go less cycles, but may have lighter weight, or much higher c rates, so it's a trade off what you need, lots of cycles or lots of discharge rate for a reasonable price. A123's(lifepo4) have both high c rate and high cycle life, but they cost ya.
It's very bewildering to wade though this at first, but the main thing is to let your ride tell you what you need. You may only need a couple lithium drill batteries, or a big long range pack depending on how you use the ebike.
As to the 40 cell pack you mentioned, that is not too bad for number of cells for a 20 ah pack. The known to be probematic 18650 cell lifepo4's on ebay have like 150 or so cells. That's way too many spot welds to depend on. My ping 36v 20 ah pack has 48 prismatic cells.