The good thing to know here is that these are ultracompact strollers (read more here). They are not made for multiple kids and superb durability, they are made for occasional use on holidays, during shopping, easy walks, etc. To expect them to be heavy duty for a few kids is not realistic, to be honest.
Grass and cobblestones, the same story. For that, you need larger wheels and stronger, a bit bulkier chassis. The frame with multiple points of folding will get loose and rattly quick when daily on rattly terrain.
Maximum comfort for the child means a more upright seat, which no ultracompact has, a lot of space, which no ultracompact has, and a large hood, which almost no ultracompact has. This is important to know, really, these strollers are not comfort strollers. The more comfy here is the Jackey, as it is more padded, but other than than, this is no comfort for a toddler for multiple hours, none of these two. And the leclerc is also not upright, unfortunately. Both buggies lie flat enough, not 180 degress, but close to 170, which is ok for smaller babies.
My own advice is something a bit bugger. From the Baby Jogger City Mini Zip/Baby Jogger City Mini2 to an Oyster Zero, TFK Dot 2, Gesslein S4, ABC Design Avus, these lightweight mid-size strollers will be comfy for the child, not the travel, holiday strollers made with only one thing in mind - fold small enough to fit the overhead compartment.
I know I was harsh, but I am sorry, you want your baby to be in comfort, but go for the strollers that are only comfortable to store in your car boot, not roomy and functional enough for daily use. Try to reconsider your priorities, as for a 3mo, is not just occasional, I would get a more full-featured stroller.
P.S. And which is better from these two? I'd go for Jackey, because of the padding. But soon, the Miley2 will be out, and that will be a tiny bit more spacious.