I just tested out 4 pair of flats (utilizing running warehouse free returns).
Nike Streak LT3 - It is super light, firm, a lot of road feel. A "track spike for the road" is the most common description, and i'd say its accurate. The rounded outsole feels awkward to me at slower paces. It's meant to race in only. Most everyone would not want it as a regular trainer.
Nike Streak 6 - Light, firm ride, but doesn't have that low to the ground feel some want in a flat. It still feels fast and responsive though, despite the thicker sole. Plenty fast for 5k, but you can tell they would really shine at HM and Marathon where you want some protection. The 8mm drop is forgiving on the calves (I have chronic achilles problems). Probably the best option of the 4 as a trainer.
Asics Hyperspeed 7 - Light with a soft midsole. Your foot really sinks into them on landing making them feel really comfy. My first run in them was a slow 8 miles, and they felt great. But then I did a tempo and decided they are too soft for a racer, and felt less responsive than the others. Durability as a trainer is probably not great, but they are also much cheaper than most.
Adidas Takumi Sen - Has the same cushioned yet responsive feel of the other boost shoes. I would have bought them if not for the price. The adios would be better for training though, mostly because of the added durability. The nubs on the outsole of the takumi would wear down fast.