Yeah, totally different. Consider the fact that the volcano on the Big Island is still active and is the youngest island. Oahu is the oldest. Also, Diamond Head is interesting for: (a) the views of Honolulu; and (b) was also an observation platform during WWI and later so it has both pathways, staircases and tunnels so is interesting just for that. The terrain is very different given the age differences. Volcano National Park is like a moonscape - barren and hardened lava formations that are very distinctive. Diamond Head is now covered in vegetation. In most of Volcano National Park (at least the areas we were in), the terrain is relatively flat where you are able to walk. Diamond Head is an uphill climb.
It took us 45 minutes to climb Diamond Head. At VNP, we walked 3 miles to try to get close to the active lava flows on a wide gravel lined pathway (which was flat but the path sort of undulates up and down over the old flows). We turned around after we realized when talking to other people that we had about another 3 miles to go. It took us about 1.5 hours to go that first 3 miles - incredibly hot, no cover, high winds (we were already tired - we'd also walked through the lava tubes, hiked over a wooded area to get there from a farther parking area, walked through the sulphur flats, wandered around the caldera observation point). Diamond Head was more shaded and a bit misty (although it could have just been the weather that day).
I actually think that doing both is really important if you are at all interested in volcanoes. You get to see the beginning of them and what happens over the passing of time. Great context.
How much age factors into the ability to get to the top of Diamond Head is difficult to say. DH and I are both in our 50's but we're both weightlifters. Having said that, we're not terribly good endurance-type people but we still made it to the top in the average amount of time. Admittedly, there was a huge amount of grumbling on my part when I saw some of the stairs (one flight of 72 and one flight of 99) and I had to focus on being slow but steady to make it. The pathways through the vegetation can be slippery and there's lots of roots and rocks that mean you have to watch your step. I think people in average good health who are moderately active will be fine especially if you take your time. Still, even if you only make it partway up, the views are great.