I have about 50/50 Peco and Hornby on my layout and to be honest I don't see any real difference in running quality with regular staright and curved track. I actually find Hornby flextrack slightly easier to work with laying it on the baseboard.
Points are another thing though.
Contrary to what most people seem to recommend I only use insulfrog points on my layout and have a mix of both Hornby and Peco setrack points. The Peco certainly seem to be of better quality, are sturdier and give a more positive throw action. But I have'nt noticed any difference in running. One issue is that if, like me, you use older running stock you can have issues with older wheelsets bridging the insulated frog and causing a short. This only happens with the Peco insulfrog, as the plastic section is smaller.