I'm debating on whether or not to get a subscription or cartel coins my personal thought is that I can get all the things I need with cartel coins and they won't expire. Is there anything that I can only get through subscription?
The full breakdown of privileges for subscription vs. preferred status vs. free to play is on swtor.com.
Most restrictions can be bypassed with cartel coins to access otherwise unavailable items (e.g. purple quality items) but preferred status and free to play players have a credit cap which prevents you from purchasing items that sell for higher than the credit cap. For the same reason, some mission rewards are not accessible to preferred status and free to play players.