The difference between the DS and DL, in viewfinder brightness, is not that much, Popular Photography even commented that the DL looked brighter because of a new focus screen. As for the tunnel vision, the DL is close to the DS (unlike the Nikon D50 or Canon Rebels which have very small viewfinders)
I have a DL and I used a DS extensively, there isn't much difference.
BTW I have used the Sigma 18-125 on a Canon 20D, it is an adequate lens but not very sharp at the longer end. The guy with the 20D also has a Sigma 70-210mm f2.8 (very nice lens) and there is an obvious difference between the sharpness at the same focal length and aperture.