How old is the current roof? If you would be replacing it anyway in the next few seasons, I would go ahead and do a full replacement. If it is relatively new, do you have or can you secure identical replacement shingles? What is your most important issue, cost or appearance. If cost, just replace the damaged area with shingles which match as close as possible. If appearance is your greatest concern, then replace (if you can't perfectly match) at least the entire side with the damage.