Womb chair and ottoman: Buy new or vintage?  

Vintage is less expensive...
...but there's no major design or construction difference. As far as I can tell, they're making new chairs basically the same way they made the old ones.
Of course, there are small differences: The frame welds were slightly different in the very early days, and I'm pretty sure that the glides changed a couple times over the years. New foam is different (better) than old foam. Some old fabrics are no longer available, but there are lots of new fabrics. Oh, and you can buy smaller sizes now.

I would worry about the latex...
I would worry about the latex foam in a vintage chair. It has a shelf life and even if you can find a chair with foam that has not hardened, it will happen sooner or later. Replacing it is not easy or cheap.

The vintage shells
were made of fiberglass and when they replaced the material with a new, more envirnomentally sound fiberglass that's different, they were unable to make the loveseat.
I haven't heard that the quality of the plastic shell is different in the chair and ottoman, however.
I would guess that the metal frame is sturdier nowadays. My friend who reupholstered my Grasshopper chair and ottoman has done Womb's in the past and they are not extremely difficult to do, so if you can find a vintage one in decent condition, you can get it redone.

Am I a broken record?
Vintage is better, vintage is better, vintage is better...
Well, in many cases, in my opinion, it really is. It's that same old expression, "they don't make them like they used to".
With regards to a womb chair, I don't know if it makes a huge difference, the new ones seem OK, but having seen different generations of them stripped to their fiberglass shell, there really is a difference. The old ones are much more substantial. Same with other Saarinen pieces. I recently had the vintage Saarinen side chairs we use in our kitchen reupholstered and when they were "naked" I was shocked at how much thicker the fiberglass is than current production. They don't have as much flex as new ones obviously, but I'm big and I like not being concerned that I might damage them by leaning back too hard. Same with our womb chair, it just has more heft than a new one. Not that I move it around really.
I'd think that you could still get a vintage womb chair and reupholster it for considerably less than a new one costs. And I know that you can get a second-hand womb chair and ottoman in good condition for less than you'd pay for a new one.
Seems like an easy decision to me.

Easy to check
My vintage Womb weighs 38 pounds with its cushions. Can anyone weigh a new one for comparison?