Main difference between the aluminum vs Ti is cost, but if you are going to use the light med/high mode for extended time, aluminum offers better heat management. Ti is a beautiful easy to maintain finish though. I have the XP-G2, very nice creamy tint with good throw. Some have reported greenish tint with the XM-L. Illumination Gear is a good U.S. supplier: http://www.illuminationgear.com/14322/index.html
No momentary, it’s a reverse clicky; no memory either, if turned off for a second or so it reverts back to low. But if it’s a concern then in tactical mode the strobe is fast enough to be on.
I have an XPG S2, neutral white makes things look nice and close enough to natural light for colors that matter. Nice wide usable spill with enough throw to see what’s up ahead. Illuminationgear.com for purchase, great people, super fast free shipping on already good prices.