So when I bought all my dive gear I was basically given a steel HP 119 for free.
I would love to have a second tank so when I take a day off work to go diving I could go for 2 dives.
Steel is good. I have maybe 25 steel cylinders and one aluminum one (a 19 cubic foot pony cylinder).
HP119 (or HP117 as Faber calls essentially identical cylinders) cylinders are great if you are OK handling the weight and bulk. I use HP120s which weigh the same and they are heavy -- but -- it's great having the extra air.
While there is something to be said for having identical cylinders, you may find that an HP100 is a better overall choice for your second cylinder.
Much depends on your body size and to some degree it depends on the kind of diving you do.
If you shore dive:
* An HP117 will allow you to make longer dives. In shallow conditions you might get 75 minutes or more plus a reasonable reserve. This is important particularly if the shore access is quite a distance away from what you want to see or in situations where you want to make a circle around a lake or a little bay or whatever.
* Shore diving into deeper water your NDL will be limiting on air but you will have a larger reserve that you can use at the surface
* If you are diving with someone else then that person's air supply may limit your overall dive duration making the extra air you are carrying less useful
* You may find that you prefer one long dive on a 117 to two shorter dives on an AL80
If you mainly dive from a dive boat:
* on shallow dives you will be limited by the "slightly less than an hour" time that nearly all boats set as the maximum time in the water and the extra air (beyond 100 cf for most people) you have won't help
* on deeper dives if you are diving air then in most cases your NDL will be limiting and the extra air won't help (again beyond 100 cf for most people)
* on deeper dives with nitrox then the extra gas will let you stay down longer, again, your buddy must also be able to match you for this to be useful