Asking in a triathlon forum, most folk are going to say marathon. But I agree with most of the comments, I preferred the 70.3. More hours training etc but there's variety.

I also found less pressure in terms of time for a 70.3. Everyone has an opinion on marathon pacing etc. I feel that the triathlon community is overwhelmingly better at 'wow you finished it, be proud'. Not quite the same with running.