If you're just starting, we recommend you get a full kit. If you're handy with a soldering iron and are looking for a fun assembly project, get an unassembled version. If you just want something that will fly right out of the box, get a ready-to-fly (fully assembled) one (you only need to add a battery and RC-gear of your choice).
Remember that even with the best pilot and hardware, you will have a crash at some point in time... this is a fact. So it is really expected from you the customer to get familiar with the equipment and how to repaid/upgrade it. It is not that hard to do it yourself and our products are backed by the best online community out there. DIYDRONES.COM
Before you start any assembly, READ the WIKI... everything is really well documented there and will save you time, frustration an money.
When you want to get maximum stability, you need to get your hands dirty and tune your copter.
This discussion has all the information you need, even video to get you going.
Of course if you are in Canada you will want to buy your gear straight from us!
If you are not in Canada you can buy full ArduCopter kits at any distributor listed here.