Are the filters
http.host != "" and
http.host the same? The first one makes the filter text edit turn yellow (suggesting I may get undesirable results) whereas the second passes the syntax check fine. They seem to produce the same output.
Are the filters
The use of the NOT (!=) operator in Wireshark comes with a caveat, as mentioned in the documentation
6.4.4. A common mistake
Warning! Using the != operator on combined expressions like: eth.addr, ip.addr, tcp.port, udp.port and alike will probably not work as expected!
Often people use a filter string to display something like ip.addr == 22.214.171.124 which will display all packets containing the IP address 126.96.36.199.
Then they use ip.addr != 188.8.131.52 to see all packets not containing the IP address 184.108.40.206 in it. Unfortunately, this does not do the expected.
Instead, that expression will even be true for packets where either source or destination IP address equals 220.127.116.11. The reason for this, is that the expression ip.addr != 18.104.22.168 must be read as "the packet contains a field named ip.addr with a value different from 22.214.171.124". As an IP datagram contains both a source and a destination address, the expression will evaluate to true whenever at least one of the two addresses differs from 126.96.36.199.
If you want to filter out all packets containing IP datagrams to or from IP address 188.8.131.52, then the correct filter is !(ip.addr == 184.108.40.206) as it reads "show me all the packets for which it is not true that a field named ip.addr exists with a value of 220.127.116.11", or in other words, "filter out all packets for which there are no occurrences of a field named ip.addr with the value 18.104.22.168".
It might be that for your specific filter at hand, the current capture are displaying the same results, but it might give you a different result with a different capture
"http.host" means any packet which have HTTP hosts "http.host != "" " means any packet which http.hosts isn't empty.
How will the second one react if you do not have http.host at all (ie: non-http traffic?) you might want to check that