Have seen guidelines about using only aluminum wire on deciduous trees and cooper on conifers ........ is that critical and, if so, why...? Thanks

Decidious trees have softer bark and you can damage it when wiring it the wrong way. Aluminum is softer and somewhat more forgiving than copper wire. On junipers, you need stronger wire to hold the branches. You can use aluminum also on conifers but usually you need thicker wire. And also the color of the copper wire looks better on conifers.
I use aluminum. It's cheaper and it's anodized brown. I don't mind.

With any luck at all, the wire will be on the tree a fraction of the tree's life. Appearance of the wire really isn't that important, IMHO. Go for cheap.

For coniferous trees wire may stay on for a year or more. And most will allways be wired to stay to shape. As such, it is important to go with something that ages dark. Alu wire tends to go bright metal after a few months. Copper goes a dull brown. It also is a reason for wanting as thin as possible (Less visible), so the copper is better.

Furthermore, with increasing ramification space gets tight. Thinner wire works easier in tight spaces.

As you use thiner wire in copper than in alu, price-wise they do not differ that much in actual use.

leatherback wrote: As you use thiner wire in copper than in alu, price-wise they do not differ that much in actual use.

I find copper less forgiving, more difficult to apply.
And yes, once positioned, it stays there.

I agree with this, that coniferous trees wire may stay on for a year or more. And most will allways be wired to stay to shape. As such, it is important to go with something that ages dark. Alu wire tends to go bright metal after a few months