Hello,
It's impossible to say broadly (Have to look at specific examples), but generally if a domain is expiring then there is still a chance it could be renewed and may be more desired whereas if it's already expired it may be an indicator (Not always) that it's less desirable. Once an expired domain become available for registration and you can buy it at reg fee, then it's less likely to be as good because other domainers have already considered it and chose not to buy it (Which is why it's available for registration after it expired). If it's expiring, then there's still a chance that a lot of other buyers/domainers want to get it.
Hope that helps,