I currently have an A/B class Asp, but do I sell up and buy a T7 and get some real creds coming in or wait? I don't want the clipper as I'm a Fed
Put it like this... I Haz
Freewinder, Hauler, Viper, Cobra, T-6, Asp, T-7 and some credits that i'm building up to get a T-9.
I don't sell my "older" ships. My Viper and cobra are good fun for combat. My Asp makes for fun times exploring stuff. My Hauler is just funny, I want a white van pain job for it! My T-6 will let me go to the smaller stations. My T-7 is good for trading large amounts. The T-9 when I get it will be great for moving large amounts of stuff.
However, there is a gotcha - the Jump ranges of all these ships are different. So for trading that's a good reason to keep them. the T-9 jump range seems to suck against the T-7. So it's either huge cargo or big jumps.
As others have said, keeping the old ship also means you have a fall back plan - just imagine if you sell your Asp, get a T-7 and as you are leaving the Dock an idiot in a Cobra flys into you and takes out your ship (yes, it happened to me) I was lucky, and managed to keep my ship and make enough credits to cover the pirate incident that happened the next day. BOTH could have lost me the T-7. It's not worth flying without Plan B.