5mm pitch is a bit low for cutting wood but plenty for cutting aluminium, or at least this is true on my machine. However, I've only just started trying to cut aluminium so haven't much experience of this yet. On balance, as I said earlier, I probably should have gone for 10mm pitch but as I do a lot of small work and 3D machining, it's acceleration more than absolute speed that improves job time so in practice it's not that much of a problem - for me. There's a bunch of engineering trade-offs in there and you have to decide for yourself where the priorities are.

I have no experience of aluminium gantries. I've gone from MDF to welded steel, so can't make any intelligent comments about aluminium but there are plenty of people out there who do use aluminium (my feeling is that this is the most popular material by far for gantries) and there are plenty of build logs and other comments about it. In practice I don't think that the weight difference is particularly important (which is not intuitive, but then neither is the fact that the rotational inertia of the ballscrews is a major factor in limiting gantry acceleration) so it's down to material availability/price and facilities available for machining and assembly that will probably drive the decision. I have a MIG welder and milling machine, which was a major factor in my choice. YMMV...