Just dropping in to add my tuppeny bit (having finally finished the game about a week ago).

I went for the synthesis ending but actually found it a pretty satisfying conclusion to the trilogy. Some of the details were slightly immersion breaking (like circuitry in leaves??) but the overall arc of the ending worked fine for me.

It's not a new theme, but for me the trilogy (like all good SF) basically hammers on the relationship between us (or evolved life in general) and what we create (technology and designed life). But more than most SF the games do take this relationship through a variety of different analyses -- starting of with the role reversal of the husk conversions on Eden Prime, where immediately the organic is essentially treated as raw material by the synthetic (a theme that is made slightly over explicit in ME2), through the parent/child dynamics of Quarian/Geth where it has become clear that Geth are no longer 'designed life' but are effectively an evolution of the Quarians (which is probably worth a post in its own right), even to themes like the Krogan who are jumped out of their evolutionary trajectory and effectively turned into 'designed life' by Salarian technology who then have the presumption to genetically castrate them (building 'safety features' into their 'creation').

Coming as the wrapping up of all these themes, I found that the choice between luddite, technocrat or singularity was very appropriate -- and after I went for synthesis (suspending disbelief about the leaf circuitry...) having Joker and EDI emerge as Adam and Eve characters on a new Eden was totally appropriate. Certainly no more daft than, for example, the ending of Peter Hamilton's Nights Dawn trilogy.

Of course, I do have a second and interpretation: Shepherd gets gunned down by the reaper while approaching the beam, the scenes on the citadel are nothing more than the spastic firing of synapses as she dies, the reapers destroy us all, the cycle is completed... again. (This is probably a more believable outcome to be honest)