Hi Karen,
The difference for me would be price, time/productivity/waste and multi-use.
I’d like to have liquid lecithin on hand to slow trace with fast accelerating fo’s.
Liquid lecithin, assuming it works in the molds is more cost effective for me than the spray which has lecithin in it.
With canola/lecithin spray, I have to blot out bubbles from those molds. I assume the bubbles are from the propellant that’s in it in place of alcohol. With liquid lecithin I'd probably have less waste. I’d wipe the cavities down with a small amount of lecithin. I wouldn’t use it in the batch. I’d use it in the molds.
It's a matter of personal choice in terms of which to use-assuming that the lecithin works by itself.
You can buy liquid soy lecithin at a health food store, (where you may pay a premium for it) GNC, some pharmacies or health food sections of supermarkets may have it. Locally, I can find the gel capsules which I’m not going to fool with, but haven’t run across the liquid yet. I haven’t gone out of my way to look for it though. Vitacost.com has it for about $6.60 (32 oz.) and I buy other things from there.
If you’re happy with the spray after using it, heck, use that. Sometimes I’m willing to pay more for convenience.

And, I'm going to use my Canola/lecithin spray until it's gone. It works!
Good luck with your molds and your batch.
