There are definitely pros and cons to each option.
How old is your roof? If it’s fairly new, or at the midway mark of warranty, a repair might be okay. But if it has a 20-year warranty and it’s 18 years old, replacing the whole roof may be the way to go.
How big of an area is currently in need of repair? If you only need to fix a small area, you can probably fix just that area. If it’s more than 25 – 30 percent of your roof, you should consider a roof replacement.
What’s the additional cost to replace your entire roof?
How important is it that your roof remains the exact same color? New shingles will not perfectly match the same color as your existing ones . If your roof is highly visible and you don’t want mismatched shingles, you should replace the whole roof.
Do you have any water damage to your roof deck? If the underside shows significant water damage, shingling over them won’t fix the problem. Replacing your roof will be a better option.
Do you need to replace your gutters as well? If you need to install new gutters, it may make sense to do everything at once.