There are significant distinctions between the two, though. For instance, the Lightning payment can be immediately used in a subsequent transaction without waiting for a block confirmation to occur.
Technically, merchants could accept 0-conf transactions which would be instant in such case but that would expose them to double-spending. Instead of believing users, they can set up a Lightning Network node and trust themselves.
Merchants transact in fiat,
Most immediately convert BTC to fiat upon receipt making sure they don't lose money on btc instability.
Lightning network is even worse as the time locks insure no quick fiat conversion.
Meaning if a Merchant does use LN, they do it thru a 3rd party service that offers almost instant fiat conversion.
Otherwise , a small $1000 variance lower in BTC verse fiat could have a devastating effects on their fiat profit margins.
Merchants want stability in price , odds are Libra become more popular than LN with merchants.
If LN is to make inroads with merchants theirs needs to be price stability added.
Find a way to do that, and you could increase merchant adoption.