kids are growing - seats getting old - what's next?

After having been set with seats for a few years, I think it's now time to re-evaluate. (especially since I get the impression this is sale time of year?)
Here's my current conditions:
DD - 5 y 8 mo, 40 lb, 44 in, 15.5" shoulder
DS - 2 y 11 mo, 35 lb, 38 in, 13.5" shoulder
2005 Honda Pilot (my car, almost all car rides for kids) with
Roundabout dated 01/03 for DS
MA can't find the date but got it around 06/05 for DD
2006 Honda Civic (husband's car, takes DS about 3 short trips per week, DD very rarely)
ComfortSport dated 11-18-02 for DS
TurboBooster with highback dated 3-21-06 for DD (also used for her when traveling with a friend or have extra kid in pilot)
All seats are installed with latch and tethers.
In the pilot DD is still happy in the MA, but I'm thinking I need to move DS out of RA and into MA, so would like to get a comfy booster for DD.
In the civic I never see DS in the ComfortSport, but I'd imagine he's getting too big (saw another post on this). Due to rarely riding in DH's car, I don't want to spend much on a new seat for DS to use in that one. I figure the booster will continue to be fine for DD for occasional use.
I'd love any advise all you wise women can offer! Thanks.

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).

Thanks for the help!
It seems like the Nautilus will be the best option for DS in DH's car. It's getting great reviews here and should last us until he's nearly or completely done with seats altogether.
I'll start comparing the boosters now, but I'm wondering about the one you say is LATCH-able. I've heard things about LATCH and weight limits - are we supposed to LATCH a booster for a heavier kid? By the way, DD went to the Dr. for an ear infection yesterday and only weighed in at 38.5 lbs! Since DS is growing so fast he may exceed 40 lbs before she does - is there an issue with the MA being LATCHed with him in it? I appreciate help on this, seems I have to re-learn everything periodically as they grow!

Using LATCH with a booster is different than for a seat. Since all that is being held in is the booster, it doesn't matter how much the child weighs. The seatbelt holds the child. There is some indication that a LATCHed booster can be safer in a side impact (doesn't allow them to move as much), and it removes the risk of an empty booster being a projectile if you don't buckle it back in.

Honda has said that you should not use the lower anchors over 40 lbs with a harnessed seat, so if the child using it is over 40 lbs you should use the seatbelt to install. The MA is super easy to install forward-facing with a seatbelt, just takes a little practice.

We went out today to BabiesRUs, Target, and Burlington looking for the Monarch, Parkway, or B510 to have my DD try them out. None of the seats were at any of the stores. Burlington had the B505, but I understand the headpiece is significantly different.
I have looked at the link to the pictures of the girl sitting in each booster, but she is much bigger than my DD. The 3 seats I'm considering look so different from each other. The parkway has wide lower sides, the Monarch wide sides near shoulders (looks very wierd to me!), and the B510 has minimal sides along the body. The reviews on elitecarseats seem to imply that Britax doesn't do well with boosters, which makes me lean towards the compass.
This next seat will be her primary seat including occasional travel and sleeping. I want to make a decision soon so that I can buy during the sale if I get the Monarch, or before parways are all gone if I decide on that. Any ideas??? Thanks.