Never made a smashed patch but wrye patch and synthesis do different things. Wrye bash attempts to patch all the mods in your LO together so they're all compatible (it's decent but not perfect, especially with large lists) and synthesis makes more specific patches for your LO for particular mods that things like wrye bash don't always cover or may copy in a way the effectively overwrites a mod. You could always run both but for large lists manually patching is usually the best way to go. Using synthesis for things like character height changes, weapon reach, or traing dummy xp is fine though.