Mecha is technically possible before tanks if your lucky, but getting the mecha spiders and all the other random bits is alot of RNG to go right. Tank has 7 prerequisites, Mecha has 13, with quite a few being pre back school level, if you get the drops.
As far as role a mecha offers greater utility by having a "melee" shot attack that strips armor and high terrain damage, also superior NV and thermal to a tank, and better turning costs. The tradeoff is slightly inferior frontal and side armor, and lower resists. Mecha does have superior under armor going for it.
Mecha i find is actually worthwhile if you don't have laser tech, since only the laser tank has an unlimited ammo option, the 60mm tank in particular can suffer from having to expend limited ammo just to clear a path.