Full Spectrum Control Channel Scan is horribly inefficient, I do not recommend enabling that option.
Let's clarify that for the purposes of this discussion, we're dealing with a single system of the same SysID and WACN, with a single zone (RFFS), and multiple sites within that same zone. Other configurations like Harris that put adjacent sites into a different RFFS are a whole other topic.
If you set coverage to SmartZone, the radio will be capable of roaming between sites within the same zone (RFFS), either by looking at the list of control channels you've programmed into the system, or by making use of the adjacent site status broadcast messages over the control channel. Since most people know which sites they want to receive, they should make sure those site's control channel frequencies are programmed into the system list. However, even in the absence of those frequencies, as long as the system is broadcasting adjacent site status messages, the radio should be aware of the control channels of adjacent sites.
Note that when in Non-Affiliate Scan mode (NAS), the radio will try to cling to the first control channel it finds in the list that it can decode, and will remain on that site until the bitter end. Only when the signal gets so weak that it can no longer decode that control channel will it move along to another site.
So while perhaps not ideal if you're in an area where you need the radio to jump back and forth between sites fairly often and don't want it dwelling on a weaker site...for the purposes of @KG5PKX
's original question, where he completely leaves the coverage of Site 1 and drives into Site 2's coverage area, SmartZone with both site's control channels programmed into the system's list will work for him.
If you want to make sure the radio stays glued to a particular site, then you can muck around with creating multiple SmartNet (coverage = disabled) sites. Or if you want the radio to basically find any control channel that's out there, go ahead and leave Full Spectrum Control Channel Scan enabled, but understand that you're making the radio less efficient in finding sites with that option enabled.