If you can go to one as an Orthodox Christian, which one would be better to attend?
- Holy Thursday - 12 Gospels or
- Holy Friday - Lamentations
And here is why. While the 12 Gospels contains much more theological material, at the end of Lamentations there usually is (at least in my experience) the moment where you enter the church 'under the bier', as though you are entering the grave following Christ, which you only come out of after the procession early on Sunday morning.
So, symbolically in the liturgical dance of the Church, Lamentations is more significant. But The 12 Gospels is probably more pedagogical.