Performance between the two should be very close and might boil down to personal choice/preference, belted vs non-belted cartridge, old vs new (fad, cool factor), etc,. Regardless which one you end up with, IIWY I'd go with at least 26" 1:8" (or faster) barrel.
Good luck!
Depending on source document, case capacity on both is the same or close (93.8 H2O).
.28 Nosler -https://en.wikipedia.org/wiki/Nosler_proprietary_cartridges
.300 win Mag -https://en.wikipedia.org/wiki/.300_Winchester_Magnum
Here's a brass cost comparison for each (source: Midway USA) for your thoughts.
.28 Nosler Custom Brass: $2.72 ea.
.300 Win Mag Brass
- Hornady: $.59-81
- Nosler Custom: $1.52
- Norma: $1.19-1.40
- Winchester: $.57
- Federal: $.84
- Sig Sauer: $.76
Good luck!
Ed