First we want to make clear the the Nickel Plated locks are considered to be only TEMPORARY and while they can be bathed in without damaging the function of the lock, the plating will eventually wear off (1-3 months) and become ugly and unsightly.
Because most similar 'Day Collar' stores ONLY have nickel plated locks, they will not tell you this.  The will always skirt around the issue as well as be misleading about what the lock's metal actually is.  For this reason we offer the nickel plated locks as 'free' and our custom locks as 'upgrades'.
We don't play those games here at ToBeHis.  Only the BEST is good enough for our customers.  For this reason we have spent hundred of hours and Tens of Thousands of dollars developing and producing our custom Pure Metal Locks.  We are the ONLY store in the world to offer our Jewelry Quality, Custom locks in Solid Precious Metals like Sterling Silver and 14K gold as well as solid 316L Surgical Quality Stainless Steel.
This being said, you should always get, if in your budget, the same metal lock as your collar.  We strongly recommend NOT mixing metals.  If for some reason your budget doesn't allow for the collar and upgraded lock/clasp then the Nickel plated lock is sufficient until you can get the same metal lock or clasp.  PLEASE SEE INFORMATION BELOW FOR STERLING VS STAINLESS