Why choose?
Personally if it were me, I'd get both! haha
The two cartridges have very different capabilities, even different than your '06.
From my perspective, I'd get the .325WSM first, then if you want a .45-70 brush gun later, then add it on later. From my experience, the .45-70 is a heavy hitter, but the effective range really isn't even as long as your .30-06. Those big ol 45cal pills fly like a jumpshot. I shoot a Marlin 1985 Cowboy .45-70 out to 600yrds, but it took a ladder style flip up sight to get there. The drop for my handloads at 600yrds is about 100" below a 100yrd zero.To be effective with the .45-70, you really need to "pattern" the rifle pretty extensively, taking groups every 50yrds or so and recording the trajectory on a field card, and you need to be able to very accurately estimate ranges on the fly to reference the card.
The .325WSM should fly about as flat as your .30-06, but packs a LOT more punch, pretty close to the same trajectory with a 200grn pill as the '06 in 150grn.
I also have a .45-70 Marlin 1985 Guide Gun that I LOVE LOVE LOVE for short to mid-range hunting. I use an AR-15 style dual aperture rear sight for more rapid sighting, as well as a 3-9x50mm Bushnell 3200 on QD mounts. I can get out to 300 pretty confidently, but the hold over after 400-500 gets to be ridiculous, especially out of the short barrel.
The .325WSM will be an "easier" caliber to range with, while the .45-70 will be a LOT of fun.
For EITHER cartridge, you'll want to reload, or have a friend reload for you. Factory rounds for the .45-70 are loaded pretty mild, and BOTH rounds run about $2.50 per shot for store-bought ammo. There's a lot of powder in both, but reloading will majorly improve your accuracy, plus save you about 50-75cents a shot to boot (maybe more if your brass holds up longer).