This is definitely the best spelling.
Maybelle looks hickish and frilly.
Unfortunately my first thought when I see
Mabel is that there used to be a Blue
Peter dog called
Mabel! Apart from that, (and I think very few people would think of that!) it's a great name.
I consider it along the same lines as
Hazel, which I also like a lot.
Hazel is used on people of all ages so I don't see why
Mabel can't be too!