If you think you'll be living in your home for years to come, a new roof is a worthwhile investment. But, if you are planning to sell the house in the next five or so years, it may make more sense financially to repair the roof instead. Besides that, the new buyer may want to pick the roof color and materials themselves, so installing a new roof might even end up working against you.
If your home's roof already has 2+ layers, you'll first need to have those previous layers ripped off because most building codes prohibit more than 2 layers on the roof. In this case, it would be more of a replacement than a repair.
If your more than 30% of your roof needs work, it makes more sense financially to bite the bullet and get a full roof replacement. If it's a smaller area or areas that in total equal less than 30%, it may be best to just repair the roof, depending on other factors.
Get an estimate for the cost of repairing the roof and replacing it before deciding on what you want to do. Small repair jobs will run around $650, but if the damage is extensive, you are likely looking at thousands of dollars, pushing the final cost of a repair closer to the price of a new roof.
When you patch and repair your roof the new shingles will not match the older, more weathered ones. If this look bothers you, you should replace the roof instead of repairing it.
When water or moisture is introduced to a roof that is in poor condition, the built-up water is liable to seep into the underside and supports of your roof, weakening them in the process. If this is the case, shingling over the roof won't do you any favors – you'll be needing a full roof replacement to remedy a situation like this.