If you're planning on diving locally, I'd probably go with steel especially if you plan on diving dry. Around here, almost everyone ends up using steel tanks, but then we also dive in salt water so we need some extra weight. In freshwater you don't need quite so much.
Summertime in lakes with a thin wetsuit, then aluminum might be just the thing. Cold saltwater; steel, all the way. Drysuit? Again, I'd opt for steel tanks. It takes a lot of weight to sink a drysuit.
Aluminum tanks are generally pretty affordable, at least around these parts. I can find newer ones fairly regularly on Craigslist for $75 or less. Just avoid the old ones. They're not worth the hassle.
Steel LP72's are also quite cheap and aren't that hard to find. I've paid as little as $10 for an LP72 and they're great tanks for short or shallow dives or if you just don't use much air. Don't worry too much about the age. They're all ancient.
Modern galvanized HP steel tanks are most in demand. Particularly HP80's, 100's and 119's. Expect to pay about 1/2 to 3/4 the price you would new for these. They'd also be my first choice in tanks.
Modern LP tanks go for a little less than HP tanks. They're great if you can get big fills, but if the shops around you will only fill to the recommended pressures then I'd just pay the extra and get HP's.
What you want to avoid are the oddball tanks. Avoid the tanks with 7/8" or 1/2" necks. Avoid the old steel tanks in odd sizes like LP38's or LP50's. And I'd avoid the older steel HP tanks as well. Most were built extra heavy so as to meet DOT standards. Newer ones were made under an exemption to the rules and are much lighter. If the pressure rating on the tank is 3442, then you should be OK.