I have been to an osteopath and not a chiropractor - but I have also spoken to a representative of the General Chiropractic Council and asked him what the difference is.
The answer? There really isn't one. They call things different names (e.g. in chiropractic, you get "lesions" in areas/joints, osteopaths don't call them lesions); some are more click-happy than others - but that is within each profession as well as between both.
In fact, the only actual difference I can find is that in general, if you go to an osteopath you will pay for a half-hour treatment, and you will be treated for about that long; whereas with a chiropractor, you may be charged £40 for a 5minute crick-crack, if that's all that the chiropractor thinks is needed.
The only other difference I am aware of is that some chiros have X-ray facilities; I haven't heard of any osteopath using X-rays (doesn't mean there aren't any).
The least clicky operators are McTimoney chiropractors - they tend to do more gentle manipulations than other chiros or osteopaths.
Then you have cranial osteopaths, who don't do any clicking at all - but they might not be appropriate for your needs, if you have wonky bits.
My own osteopath (ha! sounds like he's my personal property, I wish!) is a classical osteopath - I like his way of working because he always treats the whole spine, not just the bit that you say hurts. Because it is all a balanced structure, being out in one place can affect the rest; and the bit that hurts might not be the bit that is "out". So I prefer the idea that my whole spine is being checked for misalignments. However, classically-trained osteos are becoming harder to find - BSO (British School of Osteopathy) has taken over, although some will do Master classes in classical (which my bloke teaches, he's good!)
Osteopathy has been a life-saver for me - I had transient vertigo and constant back pain - my back had 2 s-bends in it and a shunt at C7 (from a bike accident in which I also fractured my collarbone) and my pelvis was tilted up and forward on one side. After about 3 weeks of treatment, I had grown 1/2 an inch - not a lot, you might think, but it is really and all from being "straightened out". The most important thing though was losing the transient vertigo - no more walking into walls or feeling like I was suddenly going to fall over in the middle of the road - hurrah!
I agree that a recommendation is a good way to go, if you can get one - but it's not the be-all and end all - a good practitioner for one person may not be so good for another, no one's fault, just the way it is - but certainly the best way to start looking for one.
Convenience of location is important too - driving mucks up the back/pelvis - so having to drive too far after a treatment diminishes the effect of the treatment.