+1 on rent first, but the data is compelling if sharpness is important to you.
+1 on the 24-70 F/2.8L II being sharper than primes. Roger showed that the II was sharper than the very sharp 24mm T/S: http://www.lensrentals.com/blog/2012/09/canon-24-70-f2-8-ii-resolution-tests
Besides the TDP comparison link, please see:
Roger's data: http://www.lensrentals.com/blog/2013/01/canon-24-70-f4-is-resolution-tests
(Different link: All 24-70s are there -- F/2.8L II, F/4L IS, and F/2.8L there, but no 28
-70 like yours)
Be advised the jump to 82mm filters -- that's potentially an added conversion cost in UV, CPL, and possibly specialized items like ND grad rings for holders.
Also, your 28-70 Mk I has a similarly large hood as my 24-70L Mk I -- one that does not move with zooming. This allows for optimal shading for any focal length. The 24-70L II has the 24-105L style smaller hood which only covers you at the wide end. From Bryan Carnathan @ TDP:"The 24-70 L II is nicer to use with the much smaller Canon EW-88C Lens Hood (included) in place and it stores more compactly. The lens is also much easier to grasp with the hood in reversed position. The downside is that the 24-70 L II's hood only properly shades the front element at 24mm. A much larger hood is needed to shade this lens at 70mm.
The 24-70 L I and 28-70 L retract deep into their main-lens-barrel-mounted hoods as their focal length is increased, they have significantly better shading at the longer focal lengths. In other words, the 24-70 L I and 28-70 L are using an appropriate-for-the-selected-focal-length lens hood at all focal lengths while the 24-70 L II (and most other similar zoom lenses) are using a made-for-24mm hood at all focal lengths."