Below is a link to a tank specification chart:
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan
I would approach it as a systems issue as all the pieces form a whole and the whole has to be optimized with the rest of your configuration and with the diving you will be doing.
An AL plate on average is 2 lbs negative while a stainless steel plate is about 6 lbs negative.
Aluminum and steel tanks vary according to the chart.
Ditchable weight has always been highly desired in recreational diving with the smei-flawed idea that you could ditch it at depth to get positive bouyancy. In reality there is little need for that and in the real world the only real use for ditchable weight is to drop it on the surface for a diver in distress.
For any kind of technical diving, ditchable weight is not an option as you do not have the option of a direct ascent anyway either due to a hard overhead (cave, wreck, ice, etc) or soft overhead (decompression obligation) and the last thing you want is to lose a weightbelt.
One other weight option when you get to doubles is a V weight that rides in between the tanks under the bands where it is not prone to snagging on anything. A similar single tank option is a weighted single tank adapter. So in that regard, being a little light is not a serious problem as it can be cured, but being overweighted with a stainless plate in a thin wetsuit is harder to fix. IN reality, AL plates are cheap while SS plates cost about 3-4 times more and many technical divers I known own both anyway.
If you have cavern and intro to cave or any other type of technical diving in your future, it's a good idea to add the weight you need in the form of a plate and tank with the goal of being neutrally bouyant at 10-15 ft with a normal lung volume, no air in the wing or dry suit and 500 psi in the tank(s).
Tank wise, I like the exempt X, E and FX series tanks from Worthington, PST and Faber. An X7-100 for example is only about a pound heavier than an AL80 but is -1 lbs bouyant when empty compared to +4 for an AL 80, so you have a tank that is 1 lb heavier but you can carry 5 lbs less lead for a 4 pound net loss in weight you have to carry up the ladder at the end of a dive - the extra 23 cu ft of air in the 100 cu ft tank is a free bonus.
Personally, a neutral 80 would be the last tank I'd buy. A neutral 80 in comparision weighs about 1 pound more than the E or X series 100, is not much less expensive (maybe $225 compared to $250-$275) and is 1 pound more bouyant than the steel tank and you lose 23 cu feet in comparision to a 100 cu ft tank. And if you go the technical route, the bouyancy traits of a regular (and less expensive) AL80 make it more desireable as a stage or deco bottle.
AL tanks do not present problems with flash rust with hydro testing, O2 cleaning etc, and in that regard are easier to maintain, but galvanized steel tanks have a history of being extremely durable and last longer than divers (50 years is not unreasonable to expect). AL tanks for the most part do not have the same staying power. And in general, AL tanks are large and heavy for their capacity and because of that 100 cu ft tends to be the limit in Al tank capacity - at an AL 100 is a very large and very heavy tank compared to a X or E series 100.
You also have Lp versus HP and 3442 psi options in steel tanks. LP tanks are popular with cave divers as in north florida the shops will fill them to 3500 psi anyway. But...their rated pressure is at 2640 psi (2400 psis service pressure plus 10%) and for a given rated volume they are heavier and larger than a 3442 psi tank.
When you compare them, you find there is not much difference when similar size and weight steel tanks are filled to similar pressures:
PST 104 2400 +10% 106.2 cu ft, 8" diameter 26.88" tall, 46.4 lbs, bouyancy -3.3 lbs empty
(140 cu ft at 3500 psi)
PST E8-130 3442 PSI 130cu ft, 8" diamter, 26.12" tall, 43lbs, -1 bouyancy when empty
E8-149 3442 PSI 149cu ft, 8" diamter, 29.37" tall, 47.5, lbs, -1.7 bouyancy when empty
So unless you live in north florida and can reliably get 3500 psi fills in your 2400 psi steel tanks, the 3442 psi exempt/special permit tanks are probably a better option as you will get the full rated volume in a tank the same size as an LP tank with a much smaller rated volume.
