There isn't necessarily a 'best' option, it really just depends which option you feel more comfortable with and which is most convenient for you.
The upside of scanning at home is that you have control of how you scan and upload each document, but the downside is that scanning the documents yourself takes a lot of time and effort, which you can avoid by using the scanning service.
If you do scan everything at home, it's a good idea to take all the documents with you to your appointment anyway, just in case something didn't upload properly and it needs to be scanned again.