Of course there is a difference, as you yourself point out just one.
Points you don't mention#;
1. if you already have an AV (other than avast) you don't want to use the on-line installer as this would be trying to install avast on a system that already has an AV. Potential for conflict.
Bumping this five-year-old thread because I want to ask: does "AV other than Avast" include Microsoft Defender? I've never used the offline installer and anytime I've had to install Avast I used the online installer (since that's the one that it gives you from their website), and It doesn't appear to conflict with Defender - it just turns off Defender (or at least the real-time functions) and Windows Security will recognize Avast as the computer's main AV.
I know the initial post is from 2017 which is when a lot of people were still using Windows 7 and therefore Third-party antivirus like Norton and McAfee were a lot more widely used than now, so I wanted to make sure. I haven't noticed any issues with using the Online installer while Defender was still active, but perhaps for future use it's good to know.
Start your own topic. Reviving a 5 year old topic makes no sense.