Second car seat for small car?

We just took my daughter in for her 3 month well baby check and she is already 25.5 inches long, so she will be out of her Snug Ride any day now. We are planning to get a RA for the car she will usually ride in, but aren't sure what to get for our second car. It is a Saturn SC1, and her Snug Ride didn't fit in it when we used the base, so it is a challenge!

I am wondering if the FPSE would be a good choice for that car? Or if there is another good suggestion for a second car seat? Also, is the FPSE worth the $100 it goes for on ebay?

Also, should we consider trying to hold out for the Marathon over the Roundabout - is it going to be a better seat for a longer, thinner baby than the Roundabout?

I just received my FPSE today that I bought on Ebay for $100 and I'd buy another one if I needed it. This one is for my MIL's SUV since they have my son fairly often. It's worth it to me. It has the same features as a RA, (eps foam, no twist straps, front adj. etc), but I just couldn't see spending $200 for another RA for a "sometime" vehicle. My son is currently in his RA in my car (main car) and has plenty of room. I'm planning on getting a Marathon myself for when his new little sister (she's due in Oct) takes his RA. Since the Marathon will go up to 65 lbs. (if approved) and is supposed to have harness slots that height-wise are comparable to Booster seats, I will go that route instead of buying another RA for him. He'll be two and pushing 30lbs anyway. For a 3 month old, you may find that a RA is perfect for you. The only reason I'm going with the Marathon is because of my son's age and height. He's been in his RA for 1 1/2+ years and could probably get 2 more out of it at least. I also bought a Britax King directly from their website (britaxusa.com) for an astonishing $119, reg. $199. They are discontinuing the model. I love it. I bought it for my husbands F250 p/u truck. It's very, very similar to the RA. It is also quite easy to install. I installed all three carseats today with rock solid fits! Check out epinions.com for more info on the King. I was going to go with that for MIL's SUV if the FPSE wasn't going to fit my son. I'm extremely pleased with all these carseats. Hope that helps some!