I think new style exterior and old style interior would be a lot better (so to be clear i agree with dean and i think also norm).
I feel the need to note however that 'combining them' is not necessarily particularly practical and doing that job in my opinion would most likely require completely re-implementing the old style designer tools into the new ship designer system (which it appears to me doesn't share much of anything in common with the old designer), rather than getting to re-use the old code and everything being fairly easy.
The old code could reasonably serve as a reference design which already has a lot of important design decisions already made which would potentially make things easier (it is known to be a fairly solid system, so an equivalent one would presumably be equally as effective and usable without requiring huge amounts of rework to make it usable) but it would likely (emphasis on likely) be a pretty huge amount of work to actually do.
I feel the need to note however that 'combining them' is not necessarily particularly practical and doing that job in my opinion would most likely require completely re-implementing the old style designer tools into the new ship designer system (which it appears to me doesn't share much of anything in common with the old designer), rather than getting to re-use the old code and everything being fairly easy.
The old code could reasonably serve as a reference design which already has a lot of important design decisions already made which would potentially make things easier (it is known to be a fairly solid system, so an equivalent one would presumably be equally as effective and usable without requiring huge amounts of rework to make it usable) but it would likely (emphasis on likely) be a pretty huge amount of work to actually do.