hardcore is all relative...whats hardcore to you may not be to me...for example you can experience "hardcore" weather in a car campground in yellowstone in the middle of summer...what i mean is that you could get 10 degree nights to most thats hardcore and they havent even left the car campground yet!
so what you really need to be thinking about is what you will be using the bag for. like someone wrote-backpacking, car camping, climbin, etc etc....also what type of places will you realisticly be using your bag and what type of weather...
If you can only afford one bag then make sure it covers all of your bases. (remember down will last forever, synthetic-after 4-5 yrs the material inside the back starts to break down and you will experience loss of the comfort rating..for example your bag is rated at 20 degrees when you buy it 5 yrs later its now most likely a 25 to 30 degree bag..there are many companies out there. if you dont need anything fancy just something to throw in the back of the rover for summer camping with no real cold weather then a "cheap" will do, but remember with bags you really do get what you pay for. a lot has to do with construction-materials, baffle construction is key ( some styles allow way to much movement of the down or synthetic material which leads to cold spots(your all warm on your left side and yoour right side by your torso is freezing), etc etc. an example of this-some bags may feel great in the store, but in fact are prone to cold spots-the filler material moves to one area so its not evenly disperssed through the baffle.
i swear by down unless spending lenghty time in very damp, humid weather. if you consider down check out western mountaineering-they are the best, hands down for down sleeping bags, a very close second is feathered friends (both all u.s. made). western m. deals only in down sleeping bags, so they are super specialized. feathered friends deals only in down sleeping bags and down clothing. to get an idea of how a well constructed bag should look check out there websites.
i have western mountaineering 0 degree 850+down bag , gore dryloft shell that has extra down in the footbox (my feet get cold and they offer this service to customers),
my second bag is a 25 degree w/m down bag, 800+down, gore dryloft shell bag that is super light and super small.
third bag is a -40 feathered friends down bag....but thats probably way beyond what you will use yours for....
anyway make a good and informed decision-i cant tell you how many times people thought they bought the right bag because it says the brand name or the salesman told them it was good only to be totally let down at the worst possible time.