Both. I admire creativity in a chop, and if the concepts and ideas are good, I can sometimes prioritise this over the realism of a chop, especially if 'realism' comes from heavy reliance on donors and references.
However realism also includes the feasibility of the modifications in my eyes. Some chops I see with virtually every body panel redesigned, and while it may be extremely creative and look quite nice, it's just daft and I honestly don't see the point.
What I think we all notice however, is that what makes the real difference between the ranks of choppers, is not the creativity, but the amount of detail and realism that is injected into the work. In my opinion, a realistic paint job is the hardest thing to achieve in chopping, and personally, even if I have what I consider to be an epic kit design on my chop, I cannot continue it if there isn't that element of realism there.
It's all very well being creative, but if it looks like a mess drawn up in paint, regardless of how incredible the design is, to most it's not a nice piece of artwork to view. The same can be said about realism. It could look perfectly realistic, but if it looks like a boring crappy piece of photography, it's again not that enjoyable.
The key is to bring them both together, a powerful original design, that will not only stand out, but seem believable, coupled with realism, that not only looks like a photograph, but has interesting lighting and scenery. This is where the true skill lies, bringing it all together. Visualising realistic lighting in crazy conditions on an unusual kit shape is one of the most difficult things you can do.
But for those who are new to chopping, the execution is the most important, not the number of modifications or how crazy they are. I personally drew a lot more satisfaction from this, and to many others the end result is a lot more pleasing too.