I don't wonder why the OP wants an aluminum frame. After decades of riding high-end steel bikes, aluminum bikes are all I've ridden for the last 15 years or so. The quality/cost ratio is unmatched by any other frame material, and I love the utterly predictable handling of torsionally rigid aluminum bikes.

And I've always had a hard time understanding the appeal of titanium. You can build a decent frame out of it, of course. Still, while it's lighter than steel and stronger than aluminum, it's also heavier than aluminum and weaker than steel.

You don't have to paint it, though, so there's that. And unpainted titanium frames are certainly handsome.