I have them both, and really, they are both pretty easy to use - as long as you are careful with the original measurement and lining your score up for the next score.
If I were getting only one, I'd go with the envelope board. There are tons of online tutorials and youtube videos showing how to use it to make different sizes of boxes; as far as I know, there isn't a tutorial on how to use the gift box board to make envelopes
I ended up with the gift box board as well for a couple of reasons:
1) I already had the envelope board and the pillow box board. Once I have 2 of something, I think I need the whole set.
2) I was at Michaels with a coupon burning a hole in my pocket and that board was the one thing in the store that wasn't already on sale.
3) I NEEDED the punch that cut the perfect little interlocking slits in the tops of the gift box so the top flaps slotted together.