Christmas is definitely better, and you won't get unlimited apples between now and then. Someone calculated out that we'll get 56 Gold Apples between the end of Nerofest and Christmas (Nerofest itself will give us 15 plus a bunch of silver/bronze).
There are a couple reasons to use a lot of your Apples now though:
1. It's hard to spend over 100 Apples on a single event. That's a lot of real time.
2. The boxes have Medals and Chains, which are annoying to farm for Servants who need them. Tons of Servants use Chains in particular (and all the Servants who use Medals use assloads of them), so it's nice to be able to get them here. Likewise the Ghost Lanterns and Gears, if you need those.
3. It's better to get tons of XP and QP now rather than later, since you probably have Servants you want to use them on. :p Plus it's nice to split the rewards over two events, so that you'll have more room left for leftover XP for future Servants, and more ability to use them on the Servants you do have.
4. The most optimal nodes are available right at the start of Nerofest, whereas they're locked until the second half of Christmas, so you have more ability to spread it out here. This is nice for sanity reasons.
Again, Christmas boxes are definitely better, so spend as many Apples as you can there. But there's definitely good reason to spend a bunch here, too.
Good advice, thanks. I don't even have 30 apples currently, so even though it would be really useful to have the XP early, I'll probably hold off like I was thinking. I need the skill gems very badly. ---