I would say "bitcoin". Most new users would search for bitcoin and not "btc". Also when "btc" is typed in "bitcoin" is registered. A shorter URL may be better, but I would say bitcoin-keyword fits the job. Since when being searched the term "bitcoin" would be used. This also depends on the amount of effort you put into SEO.
I agree, thanks. Also do you think there is a chance that "btc" can be recognized as a bitcoin synonym by google algorithm in the future? like "usd" is for "dollars" ?