I had this exact problem the first time I tried to write a Tau list, the armies both have a lot of good options and it's about balancing what you need to take out certain things.
A perfect example, Fire Dragons. They've got slightly more expensive, but with 3+ save and run and shoot they are better than ever....but do we need them? Now with cheaper Bright Lances, Fire Prism lances and lots of D-weaponry there are some very good anti armour options to choose from.
Trying to blu tack some Wraith poses together...Eldar are so slender and fiddly.