Then I say go for a Walker. If you're just doing dressage for fun it shouldn't really matter and if competative trail riding is what you really want to do I think a Walker is a great choice. Of course, don't be disappointed if you can't teach the trot or if once you teach the trot your horse doesn't want to gait. Again that hasn't been my experience, but listening to others it can be a problem.
Maybe get an older horse that already does both? Then you know exactly what you're getting.