the AT will be a great watch for an active 'young adult' and as bigger watches are very popular with women the 38mm would seem to be the perfect choice. i'm assuming she has tried one on in an AD so knows what she wants.
as far as what movement to have, the question is, how often will she be wearing it? if she's going to wear it every day for most of the day and evening then i personally would say the mechanical is the better choice as it will maintain its power reserve because she is wearing it. the movements are extremely robust with shock resistance and very accurate. if worn daily she may have to alter the time by a minute perhaps once a month which should be no hassle especially as she would have to change the date several months of the year anyway. if she's only going to wear it for special occasions then quartz might be better so she doesnt have the hassle of setting the time and date. another thing to note is that a mechanical watch, if maintained correctly, will last for more years than we can imagine, but it is unlikely that a quartz movement will last as long. i dont know why, it just seems to be the way of things.
as for removing the bracelet, if you are a regular changer of watchstraps you'll not find it a problem if your careful. if not then get the AD to do it to avoid scratches to the lugs. you may find that she likes it on a bracelet. it will certainly look good. i believe that you can buy it on leather so you dont have the problem with the bracelet, but these original straps are usually high quality croc that may not give the 'look' she is looking for. also, if she is as active as i think she might be, a bracelet will stand up to sweat, dirt and water far better than a leather strap. she can wear it swimming, playing any sport you can imagine and it will just wash clean under a tap. the 150m water resistance is more than enough to cope with anything she can put it through.
its a great choice of watch for a young adult and i know she will enjoy it.
best
Graham