There is truth to the fact that the more lumens the light will have the more visible it will be, however there is a caveat to that. While you could find a really bright 300 lumen or so cob tail light, cobs wash out easily in daylight, but at night it will be very bright and very large looking, so in the daytime you need a very high output single led bulb type of light. This fact has not fallen on deaf ears or blind eyes at Cygolite, they make an assortment of tail lights, their brightest for daytime visibility is the Hotshot series, and most notable is the new Hotshot 200 Pro; but again it is a pinpoint beam of light, so you should combine it at night with a wide dispersal tail light like the Niterider Omega 330.
Cygolite has come out with a new series called the Hypershot 350, this light combines the pinpoint brightness of the Hotshot with a wide dispersal led for greater side illumination, and you can chose between one or the other or both at the same time.
There are advantages to using more than one tail light, more than one makes you stand out better. I have one on my seat post, on the saddle bag, and on my helmet, but during the day I only use the Hotshot 200, at night I use all three because all three have different qualities to them. The other two I use is the Niterider Omega 300, this light has unique flash patterns, it is also visible for the side, that light parks on my saddle bag; the other light is a Niterider Sentry Aero 260, that is a cob style led and is not effective during the day, but what I like about it is that it has side facing LED's and is very noticeable from the side, that one parks on my helmet, Niterider claims it puts out 260 lumens but during the day it seems more like 40, at night maybe 100, but I think cobs may register high lumens but they're too dispersed plus there is rear and side cobs, which is great for night.
If you're thinking 3 tail lights is too much and you only want one, I would go with the Cygolite Hypershot 350 because you combine the best of both worlds, a pin point light and a wide dispersal light. Later you can always put a light on your helmet if you decide you need more of an effect, and I would consider one of the Niteriders, either the Omega 300 or the Sentry Aero 260. The Aero 260 is a bit bulky, while I don't mind it on my helmet but others might.