Greyline is a good tank. If I remember right it flows over the filter media and falls back into the tank. I've never owned a super bait tank but have heard some people like them more. I think they cost more than greyline though. A lot of folks in Texas have greyline tanks because they are local.
I have a greyline (50 gallon) and a blue water bait tank (30 gallon) The Bluewater is the same as the super bait tank 2 with different aerator.
I can keep shad alive in both tanks for days with the proper salt levels and water changes
The Bluewater or Super Bait tank use a far superior filter system and are easier to net the shad from the tank.
The Greyline is a molded tank where the Bluewater or Super Bait are welded plastic. The grey line will last forever - the others will not stand up to abuse they must be cared for
I put the Bluewater tank in my boat where I catch bait - the Greyline is mounted in the bed of my truck where they stay until used (most the the scales and other debris were filtered out before the ever hit the Greyline)
The perfect tank would be a molded Bluewater or Super Bait Tank. The best of the best
I really like my grayline. Fill it with tap water, dechlorinate, couple handfuls of salt. Allows me to catch shad on the home lake and haul to pk. . As long as I keep filters clean I haven't lost many shad. Up to 48 hours in my driveway.