No, you don't need to be concerned about a mildly modded car. Once you start adding nitrous or a supercharger it is much more important. As for which brand, one is no more accurate than the other. Most use the exact same bosch o2. What you should be looking for is features, price, how easy it is to read under WOT, and if you plan on doing your own tuning, how easy it will integrate with the tuning software you plan to use.