Yeah, pretty big price difference there lol. The X650 is a badass PSU and at 80 plus Gold rated, very nice for efficiency. Its like an $80 price difference between those two so... IMO, not worth the price difference. The X650 is obviously a higher quality made PSU though.
Even though the Neo Eco do use some cheaper components, they still are Seasonic units. I have never experienced a PSU fan going bad so, I cant comment on that except to say, you can always RMA it if it happens...
I will say, I would guess the X650 fan is no doubt better than the ones in the Seasonic S12II's/Neo Eco's. I have heard at load on the Seasonics, the fan tends to get loud, but most of the time you keep it at below at that level anyways.
I dont know, I bought a Neo Eco myself, kind of unbeatable at $40AR. I am hoping its a good unit though ive never owned one. I think I am going to use it to replace my Corsair TX650 V2(the fan is a little louder than id like). Still trying to decide what to do with it. The thing I dont like about the Neo Eco is it only has 3 years warranty compared to most other Seasonic made PSU's that have 5 year warranty. It also has a lower efficiency rating than what Seasonics are supposed to have, which may just be Antec playing it safe or I dont know what.