If this is related to your still deciding on which route to Canterbury, both Victoria and St Pancras will be the origination station for the train. That's where they start. So the train will be at the platform, waiting. How long it will be waiting depends on when it gets in and if they close it for cleaning.
I wouldn't expect either in your case to be closed or cleaning. Not that they are dirty, just not that they are intercity trains like LNER or GWR or Avanti West Coast, who do that.
There are only 4 platforms upstairs at St Pancras for the high speed to Canterbury West. Trivial to find your train, but you do need to find the Southeastern area of the station, which is at the opposite end from the tube and Euston road, up escalators or lift - opposite the top end of Kings Cross Station. I'd allow 15 minutes to find your way around and grab a snack if you want. Dozens of places to grab a snack.
At Victoria there are a couple of dozen platforms, but they are all side by side in a sort of a fan shape. Reading the departure board is a bit of fun if you aren't used to it because it is so huge. The train probably will go from one of the lower number platforms - to the left as you look at the board - but you need to be very alert because the concourse will be heaving with people going every which way so you need to watch you don't get bumped into by people who do know where they are going, and the departure board is always changing so the train you are watching for will move around the board.
There's plenty of food at Victoria too. Personally for a newbie I prefer St Pancras although I have been dealing with Victoria for 60 years, and St Pancras for only about 12.
I'd want a good 15 minutes or more at Victoria just because of the congestion and complexity.