firing lines can be effective (even against mech eldar and speed freaks)
u just have to make sure u have plenty of anti tank (broadsides) and a hammerhead to drop a template on the disembarking troops
if somebody wants to sit back and shoot it out with u, let them, ull will 75% of the time
mech can also be good(me like fish of furry) but it is risky (tau shipps arent as survivable as eldar, making it almost easy for your opponent to suddenly make u have a miny mech force and a miny firing line)
overall id do a mech list, mainly cus im used to fast eldar, and relorcating your army is fun (messes with ur opponents)
but its up to u which ud prefer, and wich u can afford (tanks expensive)