Have you thought about one of the sit-n-stand models? I know some posters really love those. I thought about one of those for a while (ours will be 2 and 4 when we go this fall, although not quite as heavy as yours), but our oldest, DD, tires pretty easily and will need to sit and probably take naps.
I also thought about a colapsable double, and the kids loved it when we tried it out, but it is just too heavy and bulky for the bus. (I don't think jogging type strollers are alowed on the Disney busses from what I have read on these boards.)
I think we are going to try a couple umbrella type strollers, which recline and have a sun shade and basket, and get the stroller connector. That will give us the most flexibility I think - on and off the busses easier and if we find one kid doesn't need a stroller after all, we aren't locked into lugging a double around everywhere.
My thought on your weight issue is that I would think a stroller rated for 40 lbs. will still carry your 45 lb. child fine - at least long enough to last a week's use or so - maybe not if you were using it every day and putting stress on it for a year. Also, I would imagine stroller manufacturer's put in a factor of safety in their design and keep the weight limit low on their recommendations to protect themselves liablity-wise. Just be sure to try them out as some of the seats for the lower weight rated strollers can be smaller and your child may not be comfortable.
Sorry for the long post, but I have really been researching the stroller problems myself lately.
Cathy