Wishing tree gives a better key per coupon rate than the fruit, and gives a decent amount per day. So unless you really need a lot of cave runes, using that will probably be sufficient in the long run (due to diminishing returns)
That said, if your rune is really low right now, like no higher than lv 4 on all slot, then you might want to give it a boost, and for that the fruit is good.
Granted, ensure that the fruit will get you to next tier of rebate. For example, if you have less than 50, it's fine to get as it will get you the 50 rebate too. If you have more than 50, however, only get it if you have at least 105 keys or so (would say 112, but you also get some from wishing tree over the few days) so you can get the 200 rebate. If you have less than that (50-105), then don't get it even if you need cave rune, as unless you are willing to forfeit the rebate, you have to wait anyway which makes it moot. (same for other tiers)