rule of thumb is the bigger the reflector (both diameter and depth) the less sidespill there is and the more light is captured. good reflectors tend to be somewhat deeper than optics to provide similar beam angle, so it's a sacrifice of size and diameter for a tighter beam.
also, reflecters have a knack for making the beam much smoother than optics because they can be what is called "orange peel" textured or "spattered"...this gives them the ability to blend imperfections into a smooth beam.
the die shape (square in luxeons) is less evident with reflectors as well, you will get a more squarish beam look with optics as opposed to a reflector. the best optics i have personally seen for getting a round beam are Inova's TIROS...but they are deep enough where it would have made much more sense to put a similar sized deep reflector. although i'm not sure how the small diameter would work at that depth, i imagine it will lose some efficiency.
i've personally tried elektrolumen's 30mm optics which give a very nice almost round beam pattern with good sidespill and a small hotspot, less squarish than the Fraen LP optics, but the Fraen LP have a much smaller diameter themselves and the beam, although squarish, is larger, but brighter hotspot with almost no sidespill to speak of.
i have tried and tested an ARC-LSHF-P, which has the Fraen LP and I have been happy with it, but i have a LitFuse-modded, IMS 17mm reflector ARC-LSH-P, which supposedly has a much tighter and more efficient, and rounder beam.
most people use 17mm, 20mm, and 27mm IMS ims reflectors, and similar McR reflectors, and Fraen LP optics, Surefire and Nuwai also have nice optics/reflector. and i dont know how many people actually use the 30mm Elektrolumens, but probably not enough /ubbthreads/images/graemlins/smile.gif.
as far as sources, many can be had at the wayne's Sandwich Shoppe
http://www.anlighten.com/shop/default.php
i think the 30mm optics can only be had through Elektrolumens site
http://www.king-cart.com/cgi-bin/cart.cgi?store=elektrolumens&product=Flashlight+Hobbyist+Parts
