In my view the choice of what ingredients to use in the mix should be based on how long the plant will be in the mix before it is changed out.

If this tree will spend a few years in this mix then I would choose the gritty mix since only 1/3rd of the mix is subject to breaking down and holding more water at the expense of less oxygen. In the basic mix almost 90% is subject to breaking down.

If water retention is your concern you can decrease the amount of grit and increase the amount of Turface. Grit holds no water, Turface holds a lot.

I have some citrus trees growing in 1 part bark fines and 1 part turface and for my tastes it holds too much water. I have some more citrus coming this week and I already have their planters ready with the gritty mix.

Years ago I was afraid of the gritty mix for the same reason you are, concerned I would be a watering slave. While you probably will be watering a bit more often than with a peat based mix, I think you will be surprised how much water the gritty mix holds, particularly in a large, deep container. When in doubt, just increase the turface while decreasing the grit.

Personally I would go with the gritty mix, especially if you plan to leave the spruce in the same container and soil for a few years. If you don't think the gritty mix holds enough water, I would do what justaguy suggested, add a bit more Turface and cut down a bit on grit.

I am growing spruce, pine and fir trees in plastic containers with gritty mix and they remain outdoors year around. The roots are huge and very well developed for such small trees, so they sure do like that gritty mix!

The pine and fir bark used in the gitty mix also retain moisture so you should have no problems.