Riding experience of 21-speed vs. a 7-speed? I am 5 feet tall, age 63, weight about 110 pounds, and I ride mainly for exercise on paved greenway trails. I used to ride a lot and can go many road miles, say up to 20 miles, quite well on terrain that is mostly flat but I struggle on hills. My old Marin 21-speed from REI feels too heavy and I want a bike that is lighter weight, a little easier overall to ride, and may "help" me get up hills (a la Atlanta, Georgia, if you know what I mean...) without having to walk my bike. However, the Marin has 21 speeds and I like being able to choose gears. But I am thinking of the 7-speed Cannondale Treadway 3 which I tried yesterday and it rides like a cloud! Apparently the 7 speeds only allow you to pedal to a limit on flatter or downhill terrain, and then it sort of spins out and you have to coast. At least that is how I interpret what the bike shop guy said. I'd rather be assured I will get some pedaling exercise while flatter sections. Is this a good tradeoff? I don't want or need a souped-up bike and at $635 the Treadway seems like a nice buy.
Do you use the front derailleur?  It seems many people in reasonably flat areas do not.  If not, you are riding a 7 speed.  Take a ride and notice what gears you are using, particularly on the steepest climb and the steepest descent.  The lowest gear will be with the smallest chainring in the front and the largest cog in the back.  The highest gear will be with the largest chainring in the front and the smallest cog in the rear.  Your gear ratios come from dividing the number of teeth on the chainring by the number on the cog.  Now you can compare the ratios available on the new bike to what you are using on the old one.  If you are having trouble up hill, do not accept a bike with any higher a low gear.  If you want to pedal down hill, do not accept a bike with any lower a high gear than you use now.

If you are using all the gears you have, a new seven speed will either have a much narrower range of gears or much wider spacing between the gears.  The spacing won't be a factor 3 wider because the gears overlap.  If the new one has the same range, try shifting two gears at a time on the old bike.  Is that too large an increment?  If so, you need more than seven gears.