It depends on what your after. I like them both very much. The safest bet by far at this point and time is the 338.

Here is a little bit I picked from a post on another site. Good read with good information.

"When introducing the 325WSM, Winchester constantly compared the 325WSM to the 338 Win Mag. This makes sense since the 338WM is a popular elk cartridge and that's the target market for the 325WSM. But some folks continued to question this approach saying the .323 diameter is not readily compared to a .338.

While reading through Ken Water's massive Pet Loads tome and reaching the section on the 8mm Rem Magnum I found some interesting statistics. These were especially interesting since he used a 24"-barreled M700 to do his testing.

First, here's the factory specs on the only current Remington factory 8mmRM load:
200 gr A-Frame - 2900 fps

Ken Waters gives the factory specs for a couple loads available in 1977 that have since been discontinued:
185 gr - 3080 fps
220 gr - 2830 fps

His own chrono testing with two different Oehler chronographs revealed actual speeds with these two loads as:
185 gr - 3030 fps
220 gr - 2755 fps

Now let's look at the 325WSM using Winchester's factory load specs:
180 gr BST - 3060 fps
200 gr AB - 2950 fps
220 gr PP - 2840 fps

And my own testing of these factory 325WSM loads using 24" barrels:
180 gr BST - 3080 fps
200 gr AB - 2885-2922 fps (2 different rifles)
220 gr PP - 2745 fps

This shows that with 180(185), 200, and 220 gr bullets in factory loads the 325WSM is fully the equal of the 8mmRM - a cartridge that no one questions as being suitable for doing some serious thumping.

Certainly the cavernous case of the 8mmRemMag would seem to lend itself to some performance tweaking with handloads. But it is noteworthy that Ken Waters goes on at length about how hard it was to even match factory speeds with handloads let alone better them. Ultimately, he found, with what he terms "maximum" case head expansion, he was able to wring out the following top speeds from the 8RM:
185 gr PSP - 3023 fps
220 gr PSP - 2878 fps

I've found it is possible to duplicate factory 180 gr and 200 gr 325WSM loads without too much effort. Reaching the factory spec with 220s is not readily doable but matching the actual clocked speed of 2745 fps is possbile with max loads.

My point of all this is to show that the 325WSM stands on the verge of being overlooked due to circumstances that have nothing to do with its capabilities. With bullets under 220 grains it performs up to expectations. With 220s, it takes max handloads in an 8mRM to better it."