Both "Restomod" and "Restoration" are forms of restoring an antique or classic car to an appearance similar to when it was new. Depending upon the value and desire of the specific vehicle and the budget of the person(s) doing the restoration will determine which method is the appropriate way to go.
Resto-Modding is when a vehicle is restored using modern and/or aftermarket components and accessories in addition to original parts. A restoration is the repair and/or replacement of not only the visible items, but the core components as well. A vehicle with a high number of units being manufactured or just isn’t that desirable of a model, may be better off in the eye of the owner getting some modern modifications.
Restoration is when a vehicle is restored exactly to the original condition that it left the manufacturer’s assembly line. A restoration is the way to go if the vehicle in question is highly desirable and hold high value. Being a collector car or a rare vintage, regardless of condition, the vehicle is worth more in its original state that if modified.
When restoring a classic vehicle in preparation to be entered into specific classic car shows, every specific detail needs to be checked, rechecked and rechecked again. Nuts, bolts, wiper blades and even the specific grease used to lubricate the suspension and chassis needs to be the same as original in most "Original" classes. If this specificity isn’t the game one is looking for then resto-modding may be a more suitable direction.