I'm trying to download Lychee Slicer from Mango3D.io. I followed their suggestion by telling windows defender to allow it to download. Then I scanned it with Norton. Norton said both the website and app are safe. Should I believe Norton over Windows Defender?
I would scan the offending binary (if not too big) on Virus total. This will automatically provide a report from over 60 different anti virus products and if they detect it.
Then you can decide who to trust :)
Assuming the SmartScreen warning message was along the lines of
Windows SmartScreen prevented an unknown application from running. Running this application might put your PC at risk.
... all that means is that SmartScreen doesn't recognize the download as one that is known to be safe.
It does not mean that SmartScreen has determined the download to be malicious. You would get a more strongly worded message if that were the case.
Which AV will you trust is completely up to you. If you think that the app is OK you can whitelist it in windows Defender. (Myself I don't prefer Windows Defender, I have disabled it).
Another thing, you can see what kinds of threats can Windows Defender block. Open Powershell as Administrator and run
Get-MpThreatCatalog | Format-List * to see all threats. The list is huge, Microsoft has improved Defender (Also more annoying) much.
Virus Total is a great online platform to scan binaries for viruses (already mentioned in other answer), still if the file is to big, it is hard to upload, so I prefer IRMA as an offline alternative.