Well, I'm actually currently building with the sci-fi molds, so let me give you some hard-earned wisdom.

#301 is the fundamental wall mold of the line. It has the basic "walls" plus several blocky bits for other things. #302 Cargo bay is definitely the #2 of the wall molds. It has the nice 1.5" square "pipe" walls, plus the crates, boxes, barrels, etc. that make for great cover.

#303 (the medical bay) has only one block of any substantive value IMHO: the corner block for #301. The rest are fun for detail, but not a "boy I gotta get this one first."

For floor molds, I'd get #273. I own #270 because it was the first one released, but if #273 had come out first I never would have bothered. There's not too many places you need carpeting, and you can use it for roofs, etc.

Finally, and I know this might be controversial, but do NOT pick up #300 the Station Builder Mold. It was my first one ever and I own a dark blue one (a harder rubber than the typical green one). I advise against it not because of bubbles or whatnot, but because the mold is off. I'm sorry, but the mold's measurements are wrong. Internally they call go together, but I can line up blocks from that mold and blocks from any other and you can see the difference in length. I'll even show you photos. And it isn't a casting problem--these problems lie in their length of the long blocks. I love Bruce and his products and I know that this forum is largely positive in tone, but pass by #300.

Good luck, and it is great to have a sci-fi builder here.