Mekton Zeta Plus (and it's earlier incarnation, the Mekton Advanced Construction System) can get pretty deep in the tangled modifiers, but it doesn't *have* to, as a lot of those mods are intended to be campaign determined.
TNE's Fire Fusion & Steel (FF&S) is quite different, in that the design process is fairly linear, and you can benefit from the design you did last week by reusing that new 'turret can' you worked up. The math, while a bit dense in places, is well explained (assuming you know your algebra) and well laid out.
By comparison, T20 is plug and play.