I have a joovy Caboose (not ultralight) and also use it with my Safeseat occupied by infant and then 3 y/o who wants to ride all the time because the novelty has not worn off! But I am definitely glad its the sit n stand vs a front/back double stroller. That said, I am attempting to replace the Joovy sit n stand stroller because its just too darn big in my trunk. I have a Taurus and it's so long with the infant seat attachment it has to go diaganol across the trunk pretty much making the rest of the trunk useless. Even without the infant attachment, its still too long, the attachment just adds 4 inches or so. The SnS also takes up the entire floor area of my Suburban's cargo hold, I have to prop it on end if Im going to have it in there with anything else, then it tips over onto the other items in the cargo hold. So while I like having the infant in the carrier aspect, I am looking at double umbrella strollers and bought a used Maclaren Twin and ordered a Baby Jogger Micro Double, hoping to be happier with one of them. Haven't done trials with them yet. For times I want her in her Safeseat and are just short in/out errands or without big sister along, I bought one of the frame car seat carrier strollers. I do a LOT of "musical strollers" stroller moving/load/unloading now and just haven't found my happy option yet unfortunately.
If you're set on the SnS aspect, the research I had done before buying was that the Joovy ones were best overall. IMO ultralight vs regular doesn't matter, its a bulky stroller and pain to put in/out of the trunks and the 3 or 4 pounds doesn't make a difference worth the extra $$. But I know some people disagree think it IS worth the $$.
If the 2 twin umbrella styles fail to make me happy, my next attempt will be getting a buggy board or the Pliko P3...though I do prefer to keep the 3 y/o with a sit option for at least another year.