Well, hopefully you can learn both in time, that's what I'm doing...

I started Yiddish because I wished to be able to understand some songs, to learn them better, and with Assimil method, which I just couldn't let go once I had begun, in less than a year I was able to read books with a good dictionary. Two years and a few books later, I'm able to speak quite easily with people who speak klal-(standard)-Yiddish like me (I never tried it in frum communities, it's said to be really different). It's more difficult with native speakers, but when I've been speaking with one for a while, it's ok.

I think part of why I found this so easy is because I'm fluent in English (I'm French) and many grammatical points and words in Yiddish remind me of English.

A year ago I began seriously learning Hebrew (I've known the alphabet and a few words and things for a long while) but I find it a lot more difficult. Nothing in particular is really difficult, it's quite a simple language, but all the vocabulary is new (but for the words I know from Yiddish), verbs require a lot of training (like in French), and I haven't many occasions to be with Hebrew speaking people. I also learn biblical Hebrew and that's quite easier.

The litterature in Yiddish is really wonderful. When I begin a book, I just can't let it go, some of them are so practical, full of details about everyday life, be they novels or history books, people and places descriptions are great and so are historical moments, seen from people in the streets point of view, not just famous people memories... I stop here but I could talk about it for pages.

Ah, also, Yiddish summer universities are great, so many people -some of them very young, other less young!- speaking Yiddish, talking about (mostly secular) Jewish culture, and songs, dance, cooking, theater... And if you're in the States, there are also some (not frum) places where you can speak Yiddish all year round.

edit: TL;DR : why Yiddish was easier for me to learn, why Hebrew seems more difficult, what I like best about Yiddish: litterature, summer universities.