Well, I'm a fan of buying what you need when you need it, especially when dealing with unanswerable questions like "how old will my kid be when they are x pounds".
I would bet that her weight-growth will slow down soon, so I doubt you'll be dealing with a 60 lb 3 year old.
Unless of course that's in her family background, then never mind.
But probably things will slow down a bit, so you'll have awhile to figure out what stroller might be good, and probably by that time there will be even nicer, better strollers out there (not that the BOB isn't nice, but I've noticed strollers getting nicer and more attractive, sturdier, etc, over the years of seeing strollers, so I imagine it will continue).
Our zoo membership expired last September so we haven't been, but I'm sure my almost 4 year old can do a whole zoo day just fine, though walking back to the car is always the biggest difficulty for us. Doesn't help matters that our zoo is down a hill from the parking lot!!!
So unless her size is common in the families, I probably would just stick with the Mac for now (unless you need one for long walks/jogs, of course) and figure it out later, once you know if her rate of growth continues.