If money was no object than you could easily have an assortment of tanks (single and double) in your basement, closet or garage and pick and choose the ideal one for the job you are contemplating. I think very few of us can afford that. Therefore my recommandation would be to pick whatever will suit the type of diving you will do the most in terms of alloy and size.
My own preference is steel (for singles and doubles). I own a set of twin HP 120 and four AL tanks (two of which use as stage bottles). My intent is to trade two AL 80 tanks this upcoming summer for probably two HP 120s that I will use as singles. With steel, the extra weight you are carrying on your back (check the specs of the tank, more specifically empty buoyancy) is no longer require around your waist...be it for single or doubles. In fact, I wear no weights in fresh water when diving my doubles and I know that I am quite overweighted at the start of the dive, therefore, I carry redundancy and normally more than one. Even though I dive mostly dry (first redundancy) in doubles, I have also installed a second independent bladder (second redundancy) on my BP that I can easily remove or add and I will carry a 50 lbs lift bag (third redundancy) in a technical pouch.
I am quite certain that when diving AL doubles, not only would you have more weight on your back (that I am certain), but you would also require more weight on your belt to offset the positive buoyancy of two tanks (minus the weight of the manifold, bands and an additional set of reg) at the near empty level.
As for size...considerations would be your SAC/RMV, the type of diving you intend to do (rec vs tech) how deep do you intend to dive with that set-up, how many consecutive dives, etc.
FYI, last summer, I did dive my doubles wearing a 7mm full length wetsuit only. While I did not have my drysuit anymore as redundancy, I still had two others at my disposal to handle the possibility of a Wg failure.