I definitely think the 15 tech level smithy is more interesting and demanding than the simple system. The 10 levels per unit is something I have never tried, but I imagine it's rather messy.
However. As the game moves into the later stages of the game, when you get a couple of hundred (or a few thousand) villages, and brute force is more of an issue than finesse, it can be a royal pain to have to upgrade tech levels. Even with the relatively new upgrades-overview it can feel a touch too messy.
So I guess I'd prefer a world where suddenly the smithy system changed to simple after a set amount of time. Unrealistic, sure, but hey, dreaming is fun.