I would absolutely shell out the extra bread for a 24” screen iMac over the 20”..the screen is no less than stunning.
Go with the iMac.
I agree with Stravinskyrules. Get the 24 in. screen. I have had a 20 in. for years and last December bought a 24 in. and gave the 20 in. to my daughter to replace her old iMac CRT. Then I got another 24 in. which I’ve hooked up to a 20 in. display. The extra real estate is worth it. If you can afford it max out on the RAM too.