Of the two I would choose the 325WSM. It has a lot better down range performance than the 45-70 and an 8MM bullet is actually a pretty big bullet even by elk standards.
A handloaded 06 with the right bullet is good performer out to 500 yards. I load the 200 grain accubond to 2720 fps and it is more than adaquate to 500.
So I would choose the 325WSM to still hit hard up close and extend your max range a bit compared to the 45/70. Especially since your existing 06 would handle elk with athority at all 45-70 ranges.