Re: Space vs. Atmosphere Movement
My first question for you is how important is tactical ship movement to your game? Are ships going to be fighting other ships on a regular basis? Or are they just a means to get from one adventure site to another?
(Yes, I realize this also depends on the players, but no one can account for player action.)
If the later, you don't need detailed ship specs. If the former, you need to work out who will be fighting what where.
Also, you need to consider how realistic you want to be, and what the movement really represents. For example, in many [disclaimer]but by no means all or every sci-fi story[/disclaimer]* space battles, the relative speed between opposing ships is roughly what the example ship speeds are. Look at the Battle of Endor. The fleets were in a rough orbit of moon, giving them a speed of several kps relative to the planet, which is moving several kps relative to its sun, which is moving many kps in orbit around the center of the galaxy, etc. Yet that is not important, as the two fleets weren't moving faster then 1 or 2 kps towards each other.
SO I guess my answer is megascale flight between planets, but when ships fight their movement is rated relative to each other.
*I've put up a disclaimer! You can't say that I forgot your favorite sci-fi story that does it differently!