Flies are all about confidence and you should use what you feel comfortable with. Size is the most important matter when considering what pattern to use, with colour running at a close second. In general if the water is warm and lowish a smaller fly will be best and when the water is running a bit higher or colder consider larger flies.
If there is a bit of stain in the water a pattern with a bit of brightness about it will probably out fish than a more sombre one. Popular patterns in the area at the moment are the Allies Shrimp (and all of its variants), Stoats Tails and various tubes such as Temple Dogs etc.