Tricky one. Both my older dds have a largish sum of money saved up given from various relatives over the years.
About 18 months ago, dd1 wanted to use some of her money to buy a laptop. We did some research, shopped around etc and we advised her not to spend too much on it as there were some good models around for less money. So she bought one for about £350 (can't remember the make, it begins with A). She's generally happy with it, can be a bit slow to start up.
Now dd2 would like to do the same and she has her heart set on an Apple macbook. People we've talked to say they really are worth the money, they last longer and have resale value. Took dd2 to John Lewis yesterday, we looked at all makes but she's really keen on the macbook. She has a (second hand) iPhone 5 which she loves so I suppose she's a bit of an Apple convert.
It is her money so it's really her choice (but she has a birthday coming up so may give her some extra towards it) but dd1 might be put out that we advised her to get a cheaper one and now we are going back on our word!