The Zen Xtra doesn't have a line out. The headphone out is very clean though. Running an amp out of it yielded positive results if I remember right. The Zen also has a slightly stronger output than the X5L, but the X5L does sound a bit better to my ears, especially in the lows. The X5L headphone out does have a bit of static to it though, but you can only hear it on really sensitive IEMs such as the UM2 and Livewire, and that's only when there no music playing.
For the price they're at now, the Zen Xtra is a very good deal, not to mention it's built like a tank and you can pretty much replace anything on it with a small screwdriver. A bad firmware install is what bricked mine. If that hadn't happened I would still be using it.
I can easily recommend both. The Xtra for it's ruggedness and reliability, and the X5L for it's features, slightly superior SQ, and it's ability to be Rockboxed. It's all about what you want in your DAP.