First off, the tri-elmar is an excellent lens. Leica's don't always have to be used with fast lenses, it's only that fast wide angles and situations that require a quiet camera play to Leica's strength. These are two areas where a Leica is better than the best pro-level SLR you can buy. However, if you're going to use your Leica as your all around 35mm camera, the tri-elmar may make good sense. Three focal lengths without changing lenses, and in terms of price, three focal lenghts for the price of two. For use as a travel camera, the lens has a lot to offer. In terms of speed, Leica lenses are way sharper at f5.6 or f8 than they are at f2, and many shots require good depth of field, anyway.
That said, I ended up buying a 35mm Summicron ASPH and a 50mm Summilux for my M6 TTL. I found the tri-elmar to be too long to be a comfortable fit on my Leica. To me, it felt awkward in actual use. The focal length ring on the lens didn't seem to have a positive lock at the 28mm position. Finally, I too couldn't give up the speed of the fixed focal length lenses for the convenience of the tri-elmar. My pictures with the Leica seem evenly divided between f1.4 and f8 with very little in between. The 35mm Summicron ASPH is a good match for your 90mm Summicron ASPH in terms of sharpness and image characteristics. Personally I like the particular qualities of my 50mm Summilux (now no longer the current version) and have learned to work around its limitations. It has fantastic bokeh at f1.4 and f2 and is as sharp as any lens made at f8.
The decision is ultimately going to come down to what you're comfortable with and an honest evaluation of the kinds of pictures you're going to take, not the kinds of pictures other people take. If you bought the tri-elmar you could always supplement it later with a fast focal length in the 35-50mm range, or vice-versa. The plus side is that for the Leica M, Leica doesn't make a single bad lens.