Whether or not it will be offered in .357 Sig in the near future, I do not know. Federal arms makes conversion barrels for many pistols, but HK is not one of them the last time I checked, so unless the factory produces a barrel kit or new weapon, you most likely will have to settle for a .40.
Some of the advantages of the USP's are.
1.Accessory Rails for light/laser attatchements.
2. High Cap magazines
3. Light weight polymer frames
4. HE finish for duty use
5. Choice of fire systems and methods with modules. The most common models have the DA/SA mode, plus the option of carrying "cocked n locked" safety on, plus decocker.
6. Ambi Mag release. It takes some getting used to, but mag changes can be done using the trigger finger without shifting your grip.
The only real disavantages I see are the high prices of the weapons and a monopoly on parts and magazines.
Good SHooting
RED