I would also suggest a cross type bike as oppposed to a mtn bike. You don't want suspension if you are staying on roads. It's just going to add more weight for no good reason. Alum will be a bit lighter but really no big deal. I would suggest a Mixte type frame bike if you can find one in your price range. Real "comfort" bikes are typically really, really heavy. I know nothing about current Schwinn but I see that a local box store, maybe Sports Authority...carries some decent DiamondBack bikes. Some of which are pretty decent. In my opinion, looking at entrylevel bikes, I think a decent bikes start at around $600. Those that I've seen under that price range are bike I would not personally recommend to anyone who is actually riding regularly. If you're only going to go on 5mi rides once a month, virtually anything that moves will be OK.
Mtn bike shocks really only come into play when they hit something pretty good sized. They aren't going to help with a flat dirt road. A hybrid with wide tires would be lighter and work just as well unless you are climbing serious hills that require really low gears that might not be there on a hybrid.