The Beco Soleil is the wider base, must-use-infant-insert sibling in Beco's SSC family.
The hard specs are this: suitable for 7-45lbs/3.5kgs - 20.5kgs. You need the insert for infant use and as with all SSC, while I don't love this, I think the Beco infant insert is a better design than the Ergo; easier to secure baby in (yes, I have tried it with live babies at baby wearing meets) and very snug. The reborn doll I'm wearing is about 4.5kgs and you can see my carry meets TICKS guidelines. I found I could get a very snug fit by crossing straps in the back, but I also liked the square feeling of straight straps, and I honestly didn't notice a difference in support, just fit.
Once your baby no longer needs the insert AND can sit independently, they're ready for back carries. Again, I found this easier to use than the Ergo; the straps are not as chunky as the Tula, and I was able to swing Audrey into a back carry while keeping the chest belt secured and just removing/rethreading my arms through the straps. I had a little trouble tightening, but that was just me being unco and not realising which way the buckles ran. I especially like the safety clips - not JUST on the waist (where you need two hands to simultaneously press the middle and side catches) but also on the arms. I'm not much of a fan of hip carries in SSC, but it is another possibility.
As you can see, Audrey (currently 2.75yo and about 15 kgs) easily fits into the body of this carrier. Given that the seat is about 16 inches/40 cms wide, she's supported to mid-thigh and that is absolutely fine for a child who is walking and has healthy hips. Knee-to-knee is essential for babies as their hip joint forms and matures; it's not an absolute requirement on and on through childhood. Audrey complained that she couldn't get her arms out, but not a word about her legs!
In terms of being a OSFM carrier, this does a really good job. Waist cinches to a snug 28 inches, will let out to 60 inches. Shoulder straps are similarly friendly; they'll cinch to 23 inches and let out to 42 inches. If you are going to buy ONE carrier and your baby will be little in a mild season, this is a fantastic and affordable option. If you can't handle the insert, or can get by with a ring sling or a stretchy wrap for the first few months, the Soleil will keep you comfortable from 4-5 months right up to the weight limit.
A quick word also on Beco's integrity as a company; as more and more carrier companies are looking for innovative and unique prints to customise carriers and make SSC ever more attractive, issues of cultural appropriation have arisen and ignited discussion within the baby wearing community. Eco was recently approached about such an issue and handled the matter with absolute grace and tact. They went to far as to withdraw completed carriers from the market, issued an unreserved apology, and have established a contact to ensure that such an oversight should not occur again. In a global market, this kind of integrity and willingness to listen is really something. I've been delighted that the quality of their carriers matches the quality of their customer service.
This is my 4.5 kg reborn, placed in insert as shown below, then in the Soleil. Straps are straight.
Please excuse the uncooperative legs...
Baby's feet can dangle freely, the body of the carrier fits snugly over the insert.
I'm a weirdo. I like my chest strap low. I also find this much easier to do up - I leave it very loose, then do up like a bra and cinch.
With a big kid!