I have an almost 6 year old, a 2 1/2 year old and am expecting my third. We bought a duoglider when my 6 yo was little because we had another kid we watched a lot. When we had my 2 1/2 yo we decided to buy a Sit and Stand. If I had it to do over I would never have bought the duoglider and would only do the sit and stand. The duoglider has hardly ever been used -- it is just too big to be practical. While it does work great, I don't want to lug it places. Plus, my older child outgrew it very quickly (my kids are tall) and was not at all comfortable in it by the time he was 2 1/2.
The sit and stand we still use constantly. It is much more compact which is great for lugging places day to day and for fitting in the car for travel. It also folds very easily and is easier for me to lift in and out of the car. It can hold the infant carrier and still have room for the older child on back. Plus it will last a lot longer size wise -- my almost 6 yo who weighs 55 lbs can still sit on the back to rest while we are out or for occasional short rides if we need him to be (such as in a busy parking lot). I believe we got the "LX" version which is slightly longer -- not enough longer to be bulky but enough to be a little more comfortable for a bigger child in back. There is a belt for the back part or your child can get on and off at will as well which is good for older children.
We love it! Good luck with your purchase -- we are looking at new carseats right now for our third and it is so overwhelming!
E.