What kind of dive times do you get on the Duane with an AL80? Since that's the only tank Rainbow Reef offers, as I understand, I wonder if it's even worth doing the Duane for what would probably be 15 minutes of bottom time, vs doing multiple mornings at Spiegel Grove where the dives would be a bit longer and there's more new territory to explore with each dive.
Here's the differences between the wrecks from my experience.
The SG is much larger, but even so if there's no current and you're a good swimmer you can swim around the entire wreck in one dive but you gotta keep moving. You can spend up to 45 minutes on this wreck on an AL80 depending on SAC rates of course.
The Duane is a bit less than 2/3 the size of the SG and can easily be done in one dive, although most of the dive will be deeper so the dives are shorter.
I've always found a lot more marine life on the Duane.
If you like to do penetration, the Duane has mostly large open rooms and it's not so wide that you can't see from one side of the ship to the other under most conditions so don't have to worry about getting lost (assuming you don't go down into a dark hold or something). You can enter on one side, swim through a room and exit on the other side, go a bit further up the wreck, and repeat.
The SG has long corridors and many levels and you can easily find yourself deep within the middle of the ship with no easy access to an exit. There are no rooms (that I am aware of) that go from one side of the ship to the other as with the Duane. There's a lot of interesting superstructure in 60-80 ft above the deck that you can swim through and around. The Duane has only a crows nest that rises well above the deck. The chimney stack is gone as of last year.
You didn't ask about the Bibb but I'll add a quick word. It's almost identical to the Duane as far as structure, but it's not dived as much because it's deeper and on it's side so there's not too many places to penetrate or swim through unless you want to drop in on one side and drop down to 130-140' and burn through your gas and no-deco limits for no good reason. I've never seen much marine life on it.