AFAIK the pakiT is NOT aluminum; the 4 speed specs are on their website...it's HiTen steel and cromoly steel. If you upgrade to lighter components you can get titanium (seat mast, stem riser). Mine is a small frame and I specced out a full custom set up, it is very light at just about 19 lbs (and getting lighter as I am swapping in the titanium parts).
How do the brompton's fit you in terms of ride position? That's one major advantage with the pakiT; much more adjustable in terms of fit. Second is the ride quality. It feels much more like a regular road bike than a folding bike. You can soften the ride if you want by going to wider tires at lower pressure, using titanium mast and stem, swap in a carbon fiber handlebar, and if you really want it cushy put on a thudbuster. If it were me, I'd start with the widest tires and maybe the titanium seat mast. The other stuff you can easily swap in yourself if you later decide you need it.