Personally I don't like flat bars though.
staehpj1 is right. Drop bars aren't necessarily any lower but they give you a lot more hand positions to avoid arm fatige on long rides. I didn't mention it earlier partly because drop bars usually mean having the shifters integrated with the brake levers ("brifters" for short), and those cost at least a couple hundred more than the shifters on the bikes you've mentioned so far. It's not the only way though, as you could go with bar-end shifters or even down-tube shifters which are inexpensive and work extremely well-- they're just not in such a convenient place.
You might not be able to simply change out the bar. I don't think the diameter of the clamping area of the drop bars is the same as that of flat bars. Maybe someone else here can elaborate.
The aluminum frame is ok. Aluminum used to have a much harsher ride than the other materials, but it seems that the manufacturers have learned to partly remedy that. It's certainly not any weaker than steel which in recent years has gotten frightfully thin in an effort to compete weightwise with the other materials. In a frame breakage test report that was on a website for years but I can't find now, they simulated a hard, out-of-the saddle climbing effort to test frames. The first ones to fatigue and crack were the steel ones.
As for the Windsor Touring, is that available anywhere other than bikesdirect.com? That company is extremely irreputable. The Better Business Bureau gives Bikes Direct an "Unsatisfactory" rating due to unanswered complaints. Do yourself a favor. If you're new to this game, do not buy a bike on the internet, especially from bikesdirect.com. Although like virtually any company they do have some happy customers, you need to know that this particular company also has a disproportionately very large number of customers who are angry with them for all kinds of reasons from bad assembly job to nonexistent customer service to even false advertising and in at least one case I read about on another forum, a cracked frame right out of the box. Their "Compare at $xxxx" statements are baloney too. (In the case of the Windsor, looking over the spec.s I can say you definitely won't pay nearly $1200 that level anywhere like they claim.) You also won't get the follow-up free support you'll get from a bike shop, something you'll need if you're kind of new to all this. Buy parts online if you want to, but go to a real shop for whole bikes until you really know fitting and can do a lot of your own maintenance. The real shop may appear more expensive on the surface, but you'll save yourself a lot of grief and be more likely to succeed in cycling.