Re-entry should be an added runner. Otherwise it's a 1x re-buy tourney.
In a 1x re-buy tournament you have to rebuy immediately or be eliminated. With re-entry I assume this is not the case, can get knocked out, go have a drink and a smoke and then buy back in?
Wouldn't bother me either way, so long as it was clear what way they do it at the start of the comp and it's reasonably consistant week in week out. (The odd change in response to players demands is fine, so long as they don't do it mid comp)
If I had to pick I'd plump with the option that keeps prize pool top heavy.
There is no 'correct' imo when it comes to stuff like this, only what other people do.