I am also a lefty, and went with the magnum in part because I couldn't find a .280, in part because my dad had one too. We were handloaders then, and noticed that the case life of the 7mm magnum was shorter then the 30-06's we were also using (all loaded near max, using Sierra manual data). That may be a factor in your decision if you reload, as the .280 is a necked down '06, the case life should be similar. IMO the .280 is more efficient as well. The magnum burns a lot of powder.
Currently looking to replace my 7 mag with a 7mm-08 for general purpose deer/bear/elk at under 300 yards. The 7mm-08 is a little slower than the .280, but it can still do the trick, plus, there seem to be more options for lefties in this caliber (I'm probably going with a T/C Encore, but I'll probably end up with a bolt action also).
Between the two calibers you mentioned, I'd get the .280.