The bottomless seems to deliver shots with more air. This means that if you extract the same volume with the bottomless and the spouted, the bottomless should taste richer because you are actually extracting a shot that is more towards a ristretto than an espresso. To compare the two, you should extract shots of the same weight in the same time, not shots of the same volume. Greg Kaan pointed out many years ago that failure to account for this might be a confounding factor in many peoples opinions one way or the other.
Once you account for the above, I tend to prefer shots from spouts, which I think tend to have a smoother mouthfeel. If Im confronted with bottomless shots, I tend to stir them to try to improve them. That said, these days I stir and/or skim most of my espresso. Its an interesting subject; one of many to add to the list of things that Id experiment with if I had more time.