The Giant Roam 1 has a threadless headset and stem. The stem can be raised and lowered, though it is not as easy as a quill stem used with threaded headset. Many consider the threadless setup to be superior to the threaded headset. I have one bike with threaded headset and one with a threadless headset. The threaded headset did come loose on me once and had to be adjusted, while my threadless headset has kept its adjustment well. Both bikes you mention above have threadless headsets. If you need the stem height adjusted, it is done by adding or removing spacers between the headset and stem.
If you are borderline on height, your best bet is to find the bike you want, then try both L and XL if possible. I am 6'1" and have one XL bike and a road bike that is between an L and an XL (58cm). For me, since I am also borderline in the height category, I also get to try out both sizes to determine proper fit.
As far as the differences between the Acera and Altus derailleurs, there is not enough difference between the two to be concerned. They both are part plastic and part metal, but should serve you just fine. Both bikes look good to me, so I'd go with the one that fits you best.
I like the Giant brand. My wife has a Giant bike, and it is well built (with Altus Derailleurs). I haven't dealt with a Diamondback bike, so I can't speak for that company.
Last edited by Scooby214; 06-11-11 at 06:21 PM.