It's a difficult choice. I've done both and they were both fantastic. So the only way to decide is to list the pros and cons of each:

Yangshuo
Pros:
- Amazing, breath-taking scenery
- Great for renting a bike and cyling about, exploring little villages
- Can do rock climbing or just stroll up some of the peaked hills
- Yangshuo is a very Western friendly town with cute little cafes and Western restaurants. The breakfasts there are great (banana pancakes, yunnan coffee, natural yoghurt with muesli etc.)
- You have a wide choice of bars to go to
- Easy to meet people in hostels

Cons:
- Too many tourists
- Tourist junk everywhere

Longsheng:
-Amazing scenery
-Really clean, fresh air
- Interesting wooden houses and minority culture
- Can do treks through the valleys and hills and explore remote little villages where locals may invite you in for a cup of tea
- Not many people if you go at the right time
- Friendly locals and cheap accomodation options
- No cars, traffic, pollution

Cons:
- More hassle to get to
- Not as many dining options as Yangshuo (it's tiny)
- When trekking, local kids may harass you for money
- Locals may also try to invite you for tea, but for money