Normally I would say the mile as the training is less time consuming for someone in your stage of life (I am right there with you), but I am going to say marathon.
Based on the fact that you've already done 2 in the last year, it seems you have figured out how to carve out the time for training, and the simple fact of the matter is that you have a pretty good mile PR and no matter how well you run "for a 40 year old", you will probably be dissatisfied with the time since you will inevitably compare it to your open time.
With the marathon, you can still set a new PR and you have said that you enjoy the training itself, so it is kind of a win-win.
Combine that with avoiding the stress that doing hard 200s and 400s puts on your body, and I think the marathon is the easy answer for you.
I am biased because I basically moved to the marathon for all of the reasons discussed above.