database sharding, horizontal versus vertical, and when to pick one over the other.

hey ppl, ive been thinkin alot bout scalin databases lately. were hittin some limits w/ our single instance and sharding seems like the next logical step. but im a bit torn btw horizontal and vertical sharding. horizontal seems more common for massive scale, but vertical can be simpler if the data is naturally segmented. whats ur experiences? when did u choose horizontal, and when did vertical make more sense? any pitfalls to watch out for w/ either