It's to bad I didn't read this post earlier as I am in Ottawa until Monday we could of met. Just spent the last few weekends at the quarry Tomorrow I'm at the Connie with students, I'll be back in Feb 2012. Since you don't want to be buying equipment later on down the road might I suggest a LP 40 steel over AL.
Reasons to think about:
The LP will allow for better fills without having to search out places that require haskels to boost your O2 should you want to go straight O2 fills. This not a problem if you just go EAN blends.
The stream lining and buoyancy is I believe better, Steel is always negative, newer steels have more neutral buoyancy characteristics when empty.
Steel is more expensive then Al but my experience is that many tech divers use and prefer steel over AL. Can't say I have ever seen steel tanks floating upwards and behind the diver when slung as a stage. I can say I have seen them hanging too low, a result of sloppy rigging. AL will tend to float bottom high, not the ideal streamlining characteristic. Proper measured stage rigging of your cylinder can minimize this common effect when using AL
Depending on your environmental choice, hard overhead vs virtual overhead, Steel vs Al has both advantages and disadvantages. AL will float valve down, bottom up when dropped and secured to a line making them subject to current flow and environmental impact and possible damge to the cylinder/regulator. Something that can ruin your day just when you really need that gas at a critical time. Steel tends to lay flat and weighted thus generally more secure staying in placement even in higher current flows thus less damge potential. Proper securing techniques will keep any cylinder in place and secured on a line.
Steel when hydro tested correctly and thoroughly will permit repeated 10% overfill (2400 to 2640 psi) thus a 40 becomes 44.8 cu ft, You cannot by industry standards do a 10% overfill on AL at any time.
It appears cost, resale and buoyancy characteristics are a concern driving your decision, in any case I offer a suggestion of using steel. Down the road this may save you money in the long term and address the buoyancy concerns you expressed. Any cylinder when maintained correctly last for years, I have steel and Al cylinders from the 1970's. The bottom line is do they have the needed volume of breathing gas. It's your choice good luck