Of course the true head-fi answer is get both. If one really has to make a choice between the two, it will depend on how you spend your time and the location. For example, I have been using my portable rig as my home rig during the majority of the summer. I can't really move around the home rig all the time, hence I leave it on my room, where I live while I am studying at university. This means that it is in my interest to have a more capable portable rig than what is really necessary. On the other hand I also live in a quiet house, so basing a rig on open headphones is certainly no problem for me. This might be entirely up to you, but the increased comfort and sound of a good open headphone is certainly worth it to me, hence I got both of the alternatives.
If I was forced to make a choice with the past year as my reference, I would probably go for a portable rig. This due to the fact that I travelled and was away too much for having only a home rig to be an alternative. If I would be home more, as I suspect, will be next year, I would probably go for the home rig. I try to avoid having music as merely back ground music to tune out my surroundings, hence a good home rig would be a better choice.
But on the other hand, is there anything that prevents you from having a more modest portable rig (ER6/Super fi3/UM1 level, should be enough for non serious listening) and a decent home rig?