The OP originally asked for directions to St Just. In doing so you would presume the OP has already done their homework as that is an unusual request and that St Just is where they want to be. You don't pluck St Just (either of them) out of thin air.
It wasn't asked for a comparison of St Ives, St Just, Penzance or anywhere else.
John new, the first respondent, then confused the issue by suggesting St Ives instead.
The reality is that as a base for walking the coast path any of the three are as good as each other. St Just and St Ives share the same public transport routes.
St Just benefits from being about half way round the Peninsula.
Looking at Rick's blog of 2015 St Just is not a mining town (or village) now- that has all closed, but you will see the distinctive remains of the mining industry all around the landscape.
To suggest that Penzance is a small city is over stretching definitions. It may be by US definitions of a city (there is a whole separate thread on that, where US communities of under 1,000 people can be called a city). By UK standards it is more of a small town. Some might say a large town but that is as far as it goes