On the question as posed, UD 747 easily outclasses 777 dorm. Having said that, the 3 class 777 is significantly better than the 4 class main version - there again it's not impossible that they will allocate a 4 class on the day. I agree it's a rather odd way to select destination however.

Other factors you haven't mentioned:
There is potentially more jet lag to SIN, though that can be managed with a bit of planning.
There is a risk that EZE will be dropped as a route and coded off to IB, it's not performing well and hasn't been for some time. I can't see that happening to SIN.
Cheap ticket to SIN do come up, there is a lot of competition, so it may be better as a revenue option. E.g. ex Norway. However EZE is on the route to nowhere so it's a good use of Avios.
Also check out IB+ Avios just in case it's offpeak there or something.