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If it were me I'd get the 24" because the screen is just the perfect size. The 20 will feel small after awhile, but the 24" is incredible. As far as performace, yes the 20" will perform better, but depending on what you're doing it won't be anything major. A plus with the 24" is that ram is a lot cheaper than it is for the 20" model so you can bump to 4gb on the cheap.
 
If it were me I'd get the 24" because the screen is just the perfect size. The 20 will feel small after awhile, but the 24" is incredible. As far as performace, yes the 20" will perform better, but depending on what you're doing it won't be anything major. A plus with the 24" is that ram is a lot cheaper than it is for the 20" model so you can bump to 4gb on the cheap.

+1 for the screen
 
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