Having tripped over tent guylines in the dark any number of times, I insist on reflective guy lines:
--Gossamer Gear EZC2
--Lawson Equipment Reflective Glowire
The last is by far the least expensive because it's currently on sale and (with another 5 cent purchase) has free shipping. I have not, however personally used it as I have the other two. The two I've used (and I can't see from the specs that the Lawson Glowire is any different) hold a taut line hitch extremely well.
I especially appreciated the reflective guy lines one cold early October evening (temp about 20*F, windy, and pitch dark with no moon) when I had to wander quite a bit around an oxbow-shaped stream to find a place where I could actually get to the edge of the stream to get water. I got a bit disoriented on the way back and found my tent only because the reflective guy lines clearly reflected the light from my headlamp from several hundred feet across the meadow!