Looks like you find yourself teetering on the fence many have sat on before. You love the benefits that your steel cylinder has given you, but you see into the future that you are going to be needing a bit more gas for some of your dives. You could go the traditional route of twin steels, but then the inevitable limitations of them crop up, how to travel with them, or if not, being faced with renting what's available at your destination, etc. But consider this... I recently purchased a Z system and I think you may be an ideal candidate for one too. For the same money or maybe even a bit less than your cash outlay for tanks, bands and a manifold, you can completely revolutionize the way you dive. The limitations you are now faced with will simply no longer be relevant. The Z System would, in my opinion, be superior to what you are considering in the following ways:
1. You would be sidemounting 2 aluminum 80 cylinders, so you'd have all the advantages that go along with that right off the bat. Lighter weight, freedom of movement, enormously easier to maintain proper trim, better safety making surf entries, better flexibility in terms of equipment placement, etc. Also you won't have to worry about wet or dry, how much lead, or what your sac rate is. You'll be infinitely adjustable and adaptable to any dive at any location.
2. 2 aluminum 80 tanks will be plenty of gas for you for awhile. When you don't need 160 cu ft, just take a single bottle. When you're ready for a deco bottle, well you just clip that one in there as well. In anticipation of you reiterating that you love your steel 100, believe me I get it, but that love will pale in comparison to your love for your Z system. If you think your mates covet your tank, wait till they see your sidemount setup.
3. Travel restrictions - absolutely none. Getting to your destination with your gear will be zero headache. And no additional expense. With no backplate to worry about, the Z system packs up into a nice tight little ball in your bag. Throw in a couple of travel stage kits and you can put them on rental 80's at any dive location.
UTD sidemount is quite simply the future of diving and you are perfectly positioned to transition to it. Reef survey? Sidemount. Tech 1? Sidemount. Rebreather? Sidemount. Cave? Sidemount. Scooter? Sidemount. Sidemount. Sidemount.
My advice to you - sell at least one of your steel tanks and get a second aluminum 80. Then call Costa at UTD and order your Z system. While you think about it, check out the equipment pages at
Also feel free to call Andrew or Jeff directly. They can definitively answer any remaining questions you might have. Hope this helps.
Thanks for the advice. Elliot is planning on holding a UTD demo day one weekend so would be great to see these products first hand.