Which one of these carseat cover that make make baby more comfortable and not too hot in summer?

they're both britax. one ia marathon and one is roundabout 50. Which one do you think it has better cover ( not to make baby wet in summer and comfy)?

Strangely enough, I would look into a sheep-skin insert. It adds comfort and helps to cool the baby in the summer.

Other than that, tricks I've learned are to keep a windshield sunscreen in the backseat that you can put over the car seat to block any light from it (and block what windows you can.) Also, have a small cooler and keep ice-packs in there. When you go out and about, and leave the car in the sun for a while and the seat gets hot, come back and open up the doors to let it air out and put an ice-pack in the carseat (especially on the buckle). You can also keep wet-washcloths in there so that the baby has something to chew on or play with that helps cool them down. That worked really well with my daughter. I also bought a small car-fan that I made sure was blowing air on her on really hot days (my car does not have air conditioning...) and that helped keep the air circulating back there. I also got a little sports-water-spritzer, but a soft-spritzing water-bottle would also work, and would wet down her hair, back of the neck, back of the arms, and back of the legs, even her tummy on the hottest days. That helped cool her down if it was getting too hot.

Looking at the two seats, the Marathon is a better seat simply because it will last to 65lbs facing forward whereas with the Roundabout it only goes to 40lbs so basically when your baby is 35lbs and ready to be turned around you'd be needing to look for another car seat.

Dressing her appropriately was great, but the car can get too hot for me to sit in, let alone for a baby, so these tricks helped.

Of those two designs though, I found this Marathon with a pattern that is very light colored and would hopefully be cooler on a hot day. But again, you can avoid the heat the cover absorbs by simply covering it with a window-covering.