I use 2x red corsair AF140 fans on my 280 radiator. I know SP140 would be optimal due to more airflow since they are static pressure type but I personally went with AF140 since they are quieter. I use SP 140 on top of the rad as a push pull. So AF140 push air and SP140 pull air out on top of the case just above rad.
As far as the temperature difference you looking at 5c +/- depending on your case air flow and room temp. This is with 4.9ghz 4970k cpu idle at 26c.
So if you want quite fan go with AF 140. I have them connected to my corsair h110i gt
Also you can go with other brands that could be even more quieter.