Half Ironman or full marathon - which is harder?

Depends. How efficiently did I train, what were the conditions and course, what were my goals, etc.

I've done a number of both (8 or 10 half irons and 5 or 6 marathons) and each one was completely different.

Marathon, seriously?  Holy cats, I am never going to be able to survive a marathon.  Everything about the HIM was hard - training, racing, recovering. 

This question is usually asked at least once a year here on BT (so if you're curious you can use the search function to see previous answers/opinions).  I think for a very strong runner, who struggles with the bike and/or swim, the Half Iron might be more difficult.  I consider myself farily balanced in each of the 3 disciplines and it's a tough call.  I've run one Marathon, done two HIMs and will run my 2nd marathon in about 2.5 weeks.  I felt better the last 6 miles of my last HIM run than I did in my only marathon so based on that I say the marathon is harder.  But that also depends on pace, bike fitness etc.  So I guess I'll know a little more in 2.5 weeks.

For me, the HIM because I can survive a plain old running race a lot easier than the combo of all 3.  For a marathon, I start feeling fatigue only after mile 15/16.  For the HIM, by mile 5 of the half marathon, I'm near dying.

Depends on who it is.

For me, properly trained for both, a marathon is harder.

That's why I like them .

HIM PR: 6 hours
Marathon PR: 3:50

and the marathon was WAYYY harder, hurt more, and required a longer recovery time

Having done 2 HIMs and 3 marathons, I would say the marathon is harder for me...and it's not even close.

If you told me right now that you would pay for my entry and all expenses to race in 4 HIMs on 4 consecutive weekends, and all I had to do was finish...I'd probably do it.

Give me the same offer for 4 consecutive marathons...no effin way.  I wouldn't even do it if it was 4 marathons in 4 months.

I'd argue that a marathon is harder than an IM. I just hurt more in the last few miles of an open marathon than in a HIM or IM. A HIM or IM is all about pacing properly. In a marathon you are racing just under your LT, and the pounding on your legs and the pain in your quads is much greater. I have to walk down stairs backwards for 2-3 days after a marathon. After a HIM I can walk around fine, and it's an overall sore feeling.

That being said, I'm going into that 'pain cave' in just a few days if the NYC Marathon doesn't get canceled.

Here's one perspective.  Next spring I'm going to do two HIMs two weeks apart, and I'm considering a third a week later.  Yes, this will be difficult.   (I've done several HIMs, including some in close proximity -- I have some idea what I'm in for.)

There is no way in Hades I would consider doing the same with a marathon.  Not a chance.  12 weeks between marathons would probably be the minimum for me.

X2...any race where you fall apart is harder than the race you didn't.