For portraits and landscape, I doubt that the 5D mk3 will offer enough improvements to justify its price. I'd go for lenses, if I were shooting what you like shooting.
- 50mm f/1.4 is a not-too-costly and worthwhile upgrade from the 50mm f/1.8.
- The 24-70 II would be a lens that'll last you, it's supposed to be crazy sharp (and expensive).
I would recommend the 70-200 f/2.8L IS II, but you've already got two decent primes in that range so I'd recommend getting more primes instead. Could go for the 200mm f/2.8L (or the 200mm f/2L if you sell your wife), already covering the 70mm with the 24-70 II.