Hi Mohammad,
(1) If you are using a full frame camera, a digital M9, or any of the film options, then you might find the WATE 16-18-21 to be too wide for most uses. On the M8, with the 1.33 crop factor, the 16-18-21 becomes an effective 21-24-28 lens, which is more versatile. So your choice of body should be a factor in making your decision.
(2) The WATE is a f4.0 lens, vs. the 21mm f1.4 lens. For indoor shooting, the f1.4 can be used in all sorts of lighting conditions, including near darkness; but the f4.0 of the WATE would limit its use to outdoor daylight shooting for the most part. Anyone who has used Leicas for some time will realize that there is a need for a f1.4 lens in the stable, as that way you don't need a flash for indoor photography, and also, some would say more importantly, the out-of-focus renderings (bokeh) of the f1.4 lens, when shot at f1.4, is creamy smooth, and gives a painterly look to photographs. With f4.0, everything is in focus, and you cannot get that look, even though the lens might be superb.
(3) Are you a wide angle shooter? I have the 21mm f2.8 Asph, but don't use it as often as the 35mm or 50mm options.
(4) A lot of people are currently mesmerized by the 21mm f1.4, but once the novelty wears off, they will start to consider the size of the lens, and might regret carrying such a bulky piece of glass. Leica was originally renowned for producing the best lenses in the SMALLEST package. Lately, the lenses have become larger and larger, and the pocketability and discreetness of the original Leica intent seems to have disappeared. The need for f1.4 or faster speed seems to be the dominant mantra for today's lenses. Most of the classic Leica photographs were produced during the era when f2.0 was the fastest lens. There is something to be said about lenses with a filter size of 39mm. The 60mm filter size, though small by DSLR standards, is huge for a Leica, but seems to have been embraced by the converted. Leica photography has changed.
(5) Examine your own photography, and what and where (subjects and lighting conditions) you mostly shoot. If it's mostly in daylight, then a f1.4 might be overkill. As Mike suggested, the 21mm f2.8 might be fine, and it's a slightly smaller lens.
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A compulsion to try every lens that's out there sometimes manifests, for each lens has a unique characteristic, and chasing the best look becomes an overriding neurosis. There are some people who have dozens of 50mm lenses (yes, just one focal length). For example, the older lenses, designed by Walter Mandler, have a more artistic and dreamy look about them. The modern lenses are more clinical in their rendering. You can see the list of Mandler designed lenses here:
http://en.wikipedia.org/wiki/Walter_Mandler
My apologies if I have digressed and added confusion to your thinking process.
Best wishes,
Mango