It depends on what sealant you are using as your base protection, and how often you are likely to top it up?
The likes of gtechniq C2 and Capro Reload are pretty good stand alone sealants that offer good dirt repellancy and water beaviour. But they are also very versatile so you can use them on their own, or for topping up waxes or sealants, but as they are sealants in their own right their own properties will take over from the base sealant.
I personally love tough coat but I'm not overly sure it works all that well when being used as a top up over another product. Works great when layered over itself as often as you like and canuse it almost like a detailing spray.
If you are wanting to add a top up regularly say after every wash maybe look at a quick detailing spray that has added protection like Sonax BSD or diluted Gtechniq C2.
the options are endless.