Have you considered Tobin instead of Tobias? I like Tobin Russell It would go better with your last name. Isaac Quintin is my favorite but they are all good, names!
Quintin Russell is my favorite out of all of the names. I love Tobias as a name, but I don't like s on the end running into your surname. Isaac Russell is just ok, I just don't think it flows as well into Smith as Quintin Russell.
1. Tobias (I don't think the s issue will make that much of a difference, especially if you plan to call him Toby)
I really like all three of these! I personally would probably choose one of the two less common ones (ie not Isaac) just because Smith is so common, but that's not really a deal breaker unless you want it to be.
1. Quintin Russell
2. Isaac Quintin (Quintin Isaac would have better flow)
3. Tobias Quintin (This would flow better as Quintin Tobias esepcially with your last name)