My first impression is that you should move the Roundabout to the Civic and get rid of the ComfortSport. You have another year on the Roundabout before it expires and he will be close to the weight limit at that point. So, Roundabout and Turbo in the Civic. If he is too tall for the Roundabout, I would look at the Graco Nautilus there. It is $150, but it harnesses to 65 lbs, taller than a Marathon and becomes a booster. You would probably get the full 6 years use out of it. A 40 lb harness won't last him too long, so it is hard to go much cheaper than that. The Fisher Price Safe Voyage Deluxe might be an option - I think it's down to $130 - same size as a Marathon, but only 55 lbs. It is discontinued, so parts may be hard to come by, but he only needs a few years in it and Marathon covers can fit it.
Marathon for DS in the Pilot, so you need a booster for DD. The Britax Parkway is a good booster with side impact protection, but it is discontinued so spare covers will be hard to find. However, it is on sale ($50-70 for most colors). You could also look at the Compass B510 - side impact, folds for storage and travel. I think it runs $90-100 most places. The Sunshine Kids Monterey is supposed to be released March 5th (so says Amazon). $130, adjustable in many ways, LATCH-able. If you can wait a few weeks, there should be some reviews out pretty quickly. Recaro and Jane have some nice boosters too and there is always the Britax Monarch. Britax will be having sale in the next week or two, so if you want the Monarch, this would be the time. The Parkway is not likely to be reduced much for the sale as it is already on clearance.
It is hard to say what is going to be comfy for your DD without trying them in person. It is also hard because what is comfy now may not be in a few years (but you can hand it down).
Mama to three boys ('03, '05, '07)