COBRA is nice because you get the same coverage you had while employed. But, it is expensive.
Good reasons to go with COBRA include preexisting conditions, scheduled medical procedures during the elegible period, or expected maternity needs. The main difference you'll find is that when you go on your own you will usually wind up with an individual policy, and COBRA is typically an extension of group benefits.
If you are young and healthy, you can do better than COBRA. If you are old, sickly, , chronically ill, or have scheduled procedures, COBRA may be cheaper.
I went with an HSA when I ran my own business. They are relatively new and require a high deductable. Get a few quotes for different plans (comprehensive, HSA, HMO, etc.), then sit down (by yourself, or with someone you can trust) and go through expected and worst case scenarios for 3-5 years. You can get a good idea what your costs and risks are for each plan.