We don't watch broadcast TV here (just use it for DVDs) so my DCs, 4 and 6, don't generally have any idea of what the 'latest toy' is as they don't see much advertising. Generally I know what the popular trends are because of MN!
The way I decide is by keeping a wishlist year round to add anything I particularly think is good for my DCs. Then when it gets nearer the time I reread it and make decisions.
At the moment, whether something is new/popular or not doesn't even enter my radar when I'm choosing presents, but I know that will change when my DCs get older.