You've picked the two armies that require the most finesse to play well congratulations
. They are two of the most fragile armies in the entire game aside from Dark Eldar.
Both armies can do mech really well which is what I see you're heading towards, except they're slightly different.
Eldar mech is essentially a game of maneuvering and waiting till the perfect moment to strike then unloading hell onto the enemy and perhaps charge in for the kill, hopefully killing them off before they shoot back or assault you. Wheras tau is more a game of slowly wearing down your enemy by outmaneuvering them, shooting them and then eliminating the frayed ends. These two styles are reflected through both army lists with the Eldar being speedier than the tau while the tau have jsj.
Dire Avengers are great in waveserpents with an exarch and blade storm, unloading 30 shots at bs4 will kill most things (especially with doom). Striking scorpions, just remember they can't assault out of transports and they can infiltrate. Exarch with scorpion claw to cut through armour is the norm. Scorpions are taken in the majority of lists over Banshees as they provide flexibility and survivability. Jetbikes are really good, they are maneuverable and can take objectives while costing almost nothing and also provide support fire through their shuriken cannons (1 per 3).
The most defining point of Eldar though in comparison to tau, is that your characters actually mean something, whereas a battlesuit commander is just another battlesuit (albeit a particularly more effective one). He/She is unique and has a role no other can take.
On the tau side of things, we can do cc mildly but its almost centralized to "the seven samurai" (O'shova and his body guard which is just cool) and kroot with kroot hounds. Fish of Fury requires special modeling on the behalf of the devilfish in order to give TLOS to the fire warrior unit, whereas I just use my fish to protect my suits (playing in a group with little LOS blocking terrain). The thing with tau though is that you need railguns to kill armour, hammerheads were ommited from your list, and I highly recommend them for a mech army (Broadsides still fill this role better though). In terms of your battlesuits specialize them. If you don't you waste points on weapons which are firing at the wrong targets or are out of range and do not glue them, magnetize if possible to allow for flexibility between games.
In the end if you're in it for competitiveness, Eldar have the edge over Tau (the tau book being "dated" and not recieving alot of love from GW, there are currently no developers that like the Tau).
