I think it depends upon how padded it is and the time that you transition them. My daughter's 5 and she still uses what I guess you'd call a toddler seat that's padded on the sides. I just feel more comfortable if (God forbid) we were in an accident or the car flipped over that she had that extra padding on the side. Plus, it's pretty easy to move from one car to another.
On the other hand, if we rented a car on vacation, I don't take the car seat with us and just use the booster seat at the car rental place. At my daughter's age, I think the main idea is to booster her so that the seat belt hits her at the right place.
In my opinion, a booster seat is basically a phone book that you give a child to sit on so it raises them up to the correct height for the seat belt. A toddler car seat has padding on the sides and (in my daughte'rs case) cup holders which she loves!
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ok a toddler car seat is a booster seat except that it has a back to it. They make some toddler car seats that the back comes off (when the child is ready for it) and whala you have a booster seat. other wise you have to buy both when your child needs them. the difference of back and backless has something to do with their size, wieght, age, etc…as younger kids need the extra support of the back and neck rest of the toddler seat and as they grow they just need to be up higher with out the back of the seat. I know sounds stupid and yes most are way over padded. Under source i put some links for you of some good ones that are the toddler seat and turn into the booster. Hope they help!
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http://www.target.com/Graco-TurboBooster-Car-Seat-Delight/dp/B0015ZSMP4/qid=1236304556/ref=br_1_7/180-7844714-4723530?ie=UTF8&node=3579951&frombrowse=1&rh=&page=1

Toddler seats and infant seats have 5 point harnesses Booster seats do not they just use the regular car seat belt., some do have backs. Better helps to protect against wiplash.They do make seats with 5 point harnesses that can be made into booster seats by removing the harness. It is best and safest to keep a child in a 5 point harness as long as possible, as it is the safest in a accident. In fact they even make 5 point harnesses for children over 40 pounds(the weight limit for a toddler car seat) You cannot put a child under 40 pounds in a booster seat. It is not safe.
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car seat photos
http://images.google.com/images?q=toddler%20car%20seats%20photos&rls=com.microsoft:en-us:IE-SearchBox&oe=UTF-8&sourceid=ie7&rlz=1I7GGLD_en&um=1&ie=UTF-8&sa=N&hl=en&tab=wi
Booster seats photos
http://images.google.com/images?hl=en&rls=com.microsoft%3Aen-us%3AIE-SearchBox&rlz=1I7GGLD_en&um=1&q=booster+seats

no you can not. Boosters are for big kids, usually 4 years AND 40 pounds. Although my personal opinion is to keep kids harnessed as long as possible, as that is what I am doing with my son. After a child outgrows their infant carrier, avg. about 5-6 months, they need to be put in a convertible car seat that is rear and forward facing. They must be kept rear facing to 1 year and 20 pounds. Then they can be turned. Only now research has shown that it is much more safer to keep the child rear facing until the weight limit on their seat, which is usually 35 pounds. It doesn't matter if their feet touch the back of the seat, kids are more flexible than adults and it doesn't bother them. I know kids that have rear faced till they were 2! Lots of seats out there have a harness limit of 40 pounds but you can find a seat that has a higher weight. These are The First Years True Fit, The Britax Marathon, which go to 65 pounds or the Evenflo Triumph which goes to 50 pounds.

edit: sweet, I get a thumbs down for actually knowing about car seat safety. There's real idiots out there who put their 6 month old forward facing when it's illegal. Nice parenting!
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wait at least till there a year to turn them around in a seat.A full back booster seat is fine they last longer as they grow.as long as sit up on there own but wait till there a year it saftey reasons.I bought my seat at a sams club and its so nice n padded and cozy.remember they will b n that seat for years to come so pick a nice 1 and check the weight limit to.
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Toddler car seat provides proper back and neck care. So invest in one please to keep your child safe. Anyway, this can be used for quite a number of years. Very young children should not be put into booster seats cos their bodies are too small and will be flung out if we do emergency stops. So get booster only if they outgrow the toddler car seat (check the weight limit when you purchase).
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toddler sit is Only up to 30lbs
booster sit is 30 to 80 lbs
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I wouldn't put them in a regular booster seat until they are old and big enough. You can buy the infant seat that is 3 in 1. It's for infants, then adjusts to fit toddlers and it turns to a booster seat for older kids.

http://www.walmart.com/catalog/product.do?product_id=10759861
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Personal Experience