Took me AGES to get both. I long dismissed L'Instant as a "poorer" Héritage. I've come to the conclusion that they are equally marvellous and can be had at the same time.
They are both quite radiant, but Héritage the most because of a very sparkling bergamot, some lavender and it's more spicy with pepper and coriander. It's the most "classic" and "old-fashioned" of the two, and it's VERY Guerlain. L'Instant feels more "modern" but with the typical, perfect coumarin-wood-patchouli of a masculine Guerlain. It's also sparkling, but in a more "gourmand" and "round" way because of its anise top. It's also a bit more bitter because of some pine-y elemi resin.
I use Héritage when I want to feel very classically Guerlain and very "grown-up", and L'Instant when I want a more modern-young but still obviously Guerlain feel.
PS: Sometimes the very heavy use of patchouli in L'Instant can annoy me the day after, where it smells a bit cold and dark and actually a bit too "modernly butch". Héritage leaves you in a more bright and soft mood the day after (a bit like Habit Rouge and Shalimar do). One must bear in mind that L'Instant doesn't contain any vanilla.