Langley is a tad bit easier as there is not as much elevation gain, but the mileage is about the same if not another mile or two round trip.
The first six miles is almost flat with little gain although very tough mentally (for me at least) because I got sick and tired of that forest. Once you exit the forest, it is spectacular. Rock walls and lakes all around. I do not think Old Army is dangerous as long as you're careful. The trail is not maintained any longer and it shows, but it's not some death defying traverse. A gentleman recently died there, but I believe this was partly due to foul weather. Beyond OA, it's a bunch of use-trail and light scrambling near the top. There are multiple use-trails so don't get caught up trying to find the "right" one.
Old Army would be "spicy" in the winter and under torrential rain. I've been there in the winter...it's either fun or scary depending on you. This is the best time to go but you should probably bring Deet. Mosquito galore if you go at the wrong time.
You'll have fun at either mountain. Langley is just as nice if not nicer than Whitney.
