I'd get the 24-70mm first, because:
1. The 24-70mm may be more useful for events, even though you already have a 50mm. I am assuming events take precedence over landscape because event photography usually requires you to satisfy other stakeholders, whereas you just got to please yourself with landscape photography.
2. You can still use the 24-70mm for landscape. Angle of view may be expanded via stitching.
3. If you are serious though, just get both lah...