I also would say AP if those hours fit within 25. In fact I would say that was a very good AP role. If you need the school/club runs to be done by car then I strongly recommend you getting a slightly older (in their twenties rather than teenage) as the car insurance will be high.
I highly recommend www.aupairworld.net - we have recruited 3 AP's this way. Although you do more legwork than an agency it only costs a v small amount (about £30) compared to hundreds with an agency. The benefit of this style of recruitment means you get a much better feel for the AP and can filter more easily than you will find through an agency. The downside, if it's a disaster, you don't have the agency back up with an instant replacement. However, you can often find AP's on there looking for immediate placements e.g. if their family has fallen through.
Other things you need to think about with an AP: part of the arrangement is likely to be that they go to college to learn English - will you pay as part of your package? Check local colleges to see if their course hours fit with the hours you want the AP available for you. Will you provide a mobile? Computer/Skype/Phone access?
For 25 hours per week pocket money would usually be £55-70 per week. We pay £60 provide a mobile (but no credit), free phone, skype, computer, pay her college fees, all food, her railcard (she pays fares) and any days out she does with us.
The other major thing you need to bear in mind with an AP is that they are to live as a member of your family. This can have real pros but also you need to think hard about how you feel about having someone sharing your living space, watching TV with you, eating meals with you. If you are clear what you expect in terms of this at the recruitment stage then you are more likely to find a good match. Because really a good AP arrangement is about matching expectations.
Compared to many on him I'm still an AP novice, so I hope others come along to advise too, but in the meantime hope that gives you some pointers...