I haven't tried the boba, but I have the Ergo with the extender strap and it's comfy. I did feel like it took me a few tries to find the right spot, but I went grocery shopping the other day with LO and I didn't even feel any strain in my lower back or shoulders!
I haven't tried either of those, but I have the Beco Gemini and it works great. I'm about a size 18. I like the Gemini because it's really adjustable and my not so big husband can use it too. My friend (who is bigger than me) also tried it out and said it was comfortable. I also really like my Moby.
Re: BW, ergo, and a plus sized mama