its this simple.
if you have a 1680 x 1050 resolution or lower, then definitely get the 275 as it have faster clock speed, and will operate faster in-most games than a 280.
but if you have a higher resolution then get the 280 as more bandwidth helps rendering more pixels per sec.
even though that the 280 is a little more expensive or having a higher number that doesn't always mean that its faster, more bandwidth is needed only at higher resolution but at low you need to look for ones that have higher clock speed with a good bandwidth but as for these 2, 1680 x 1050 or lower get a 275, if higher then a 280.