Miller said:If you had to pick only one of the two which would you choose and why?
I have both, plus several other Contender barrels. The best choice really depends on what your primary use is going to be. The 357 Maximum has the advantage of being a factory round and capable of firing 357 Magnum ammo in the same barrel. The throats on the 357 Max Contender barrels are really long, which often results in poor accuracy unless you're shooting long(heavy) bullets. Another plus is that you can load 357 Max ammo with 357 Mag carbide dies.
The 30 Herrett is a wonderful cartridge if you're willing to form the cases and realize that if you forget or run out of ammo your only choice is the long drive home. Accuracy wise, I've found my 30 Herrett to have the edge over the 357 Maximum. It also shoots flatter and packs more enegry at the longer ranges.