Honestly, upgrade to a new ship whenever you're comfortable doing so. Some people like to make sure they can afford to replace it if it dies, some people like to stick with their current ship and upgrade it massively. Personally I just buy a new ship whenever I feel like it, if it gets blown up and I can't afford to replace it, then meh, I'll deal with it
The cockpits and interiors vary massively, yeah. Unfortunately all the other ships seem to have less visibility than the sidewinder, as far as I've seen; I love the window on the ceiling of the sidey.
Example: Sidewinder
vs the Adder:
(the interface looks strange on the second one because I turned the UI off for the screenshot. Unfortunately some UI elements go away and others can't be turned off)
Actually, on a semi-related note, the adder has a second seat; is there some sort of co-pilot or passenger system planned for future updates, or is that just purely cosmetic?