I've done both. My now 11 yo son has been on medication for some years, and when he was insistent that he didn't want to be on medication we experimented and he was not on meds for some of 3rd and 4th grade.
We tried the Feingold diet. There is some limited anecdotal success - but very limited. While I don't think there was any harm in doing it we did not see any significant difference and it's pretty costly to find only foods without additives, preservatives, artificial colors, etc.
I've taught sunday school & VBS and now am a sponsor in youth group - so I've had a good view of many, many kids over the years. I have friends who swear by various diets (wheat free, vegetarian) some have home schooled, etc. What I've witnessed is that although the parents think their methods are working, once the parents leave their child at suday school or youth group, the child becomes the ADHD kid that they really are. It's as if they "behave" as mom wants them to when she's around - but it doesn't stick. I just spent president's weekend on retreat with 50 middle schoolers from age 11 - 14. Trust me - the kids on the special diets whose parents think they don't exhibit ADHD charateristics still do as soon as mom drove out of the parking lot and we all got on the bus. These kids want to make mom happy - but they are still who they are.
My son has always had a tough time paying attention and taking on tasks that involve multiple steps or extended periods of concentration. He does MUCH better when he can work on something that he's really interested in - or when the teacher encourages him and has set high expectations. He has always risen to the expectations set by a good teacher. I've seen him sit down and write out 8 pages about the gold rush in one sitting - becuase his teacher expected him to be able to do it and he liked the subject. But his teacher was a special ed teacher who knew how to work with him to get the best out of him.
But through all of this - he's ALWAYS done better when on medication. He takes a very low dose, it's time released to last 7 hours - just enoguth to get through school & homework but not long enough to affect his sleep.
In the years that he didn't take medication he had a much tougher time in school. He still "got by" but he didn't excel, he really struggled and only resulted in less than average work. In the years when he's been on medication he's done so much better - his teachers have enjoyed having him inclass, he's been able to do very good work and is gratified by seeing the results of his hard work. He's done better socially, in athletics, etc.
I am not saying that medication is neccessary - what I'm saying is that it enables your child to be the child he is meant to be. He can take advantage of his skills, he can concentrate and not disrupt the class or the group, he can defer gratification becuase he's seen that there's a good outcome when he works hard.
If I had to go back and do it over I'd not hesitate to use medication. My illustration is this - if your child was diabetic wouldn't you give them insulin?
There are many highly successful people who work with their ADHD - there is something about a person who's got lots of energy and personality - if they can correctly harness it. There are great books (the gift of ADHD, etc.) to read that show you that this can truly be a gift. These kids are something else - I mean that in a good way. There are also books by Mel Levine and Cynthia Tobias that help udnerstand the different way minds work, learning styles, etc. they may be helpful as well. I personally cried when my son's teacher suggested he was ADHD - and now - 6 years later I see it as something that will one day help my son now that he's learning how to harness his creativity and energy.
Good luck making this decision - I understand your hesitence becuase I was not convinced either and delayed putting him on medicine and then stopped it for nearly 2 years... and I think I did him a disservice and wonder how much more he could have come along in that time. But God works it all out for good - so I'm counting on His grace to smooth out my errrors over the years - cuz I know there have been enough!
Best of luck making this decision.