It really is a personal decision and depends completely (obviously) on the childminders and nurseries available locally. I agree that you need to visit a few, speak to other mums in your area and get a feel for what is out there.
Lots of people will advocate a childminder for a 6 month old, for one to one care etc. My DD started at 12 months at a nursery but it had the most amazing baby . My H and I are lawyers with no other support locally so if ever we'd both been due in Court and a childminder phoned in sick, we'd have been really stuck. We thought there was less likelihood of this with a nursery. Also in my case I thought that she'd do better at nursery - I think there is more opportunity for age appropriate activities, wouldn't have to do the school run for the childminders' other mindees, easier to stick to their routine in a nursery, better long term provision (i.e. wouldn't have to change later on). Thats just my view however.
I don't necessarily agree with others that nurseries are more flexible - whilst nursery is open from 8am until 6pm, that is rigid and there is a penalty at our nursery of £11 for every 15 minutes that we are late. Similarly there have been some days when it would have been easier to drop earlier, but can't do that. Having said that, some parents have private arrangements with the nursery ladies so e.g. I know of one nursery worker who collects a child enroute, but if flexibility is an issue, childminders are usually more accommodating.
Similarly I think nurseries are more expensive than childminders. Here my DD's nursery is £65 a day - most childminders are £4 an hour (max £5). Some childminders will also charge a daily rate for 2 children which is only just more than 1 child, whereas nurseries may offer a 10% discount for a 2nd child, but it doesn't really make a dent in 2 lots of fees. If you're planning on another child therefore and want continuity of care for Number 1, might be worth considering a childminder.