I will admit that I am not up on the latest and greatest inkjet technology, but if you are talking about impact, I have never seen a print that surpasses Fuji Supergloss. When it comes to labs, they are not all created equal and therefore the prints are not equal, regardless of the equipment. One of my local labs runs the same Chromira printers as Westcoast Imaging, but can't match output quality by a mile. Another local lab (Photo Craft) runs a Lightjet 430, which I have been told is somewhat inferior to a Chromira, and their print quality is the best of the three. A Lightjet or Chromira is inferior to a new inkjet in pretty much everything you can measure, but if you want huge prints that blow people away, they are tough to beat.
All that said, I am not familiar with the inkjet papers recommended above, but I'm not sure I would want to deal with a 50x70 print on Supergloss. That stuff is a little fragile and is hard to work with (it's basically like Cibachrome), but it is absolutely stunning.