Two GTS250s will be better than a GTX285, let alone a GTX280. A GTS250 is equal to a 9800GTX+, which in turn in about 15-20% better than a 9800GT.
A GTX280 is equal to or slightly outperformed by two 9800GTs in SLi. So two GTS250s would blow a GTX280 away.
As for issues of power and heat, the GTS250s will produce more heat and use up slightly more power, but the GTX280 is an older 65nm card and is power/heat inefficient compared to either a GTS250 or a GTX285.
So in terms of heat/electricity vs performance, the 2xGTS250s will win. And in games that are not SLi compatible, a single GTS250 would be more than enough since they're going to be older games.
Get the two GTS250s!