What is your budget?
I'm not a Mac user but here is what I see at the Apple refurbish site for $40 more for a iMac then a MacBook You get a 2X faster processor, more pixels on a 21" display, twice the ram, Gforce graphics with 1GB VRAM and twice the storage but its disk not solid state.
I would go with the iMac over the MacBook with small high resolution display with fewer pixels half the ram and half the processor but with solid state storage. A large MacBook not Air would compete with the iMac but not be as portable as the Air.
Neither of these are a killer machines when it comes to Photoshop http://blogs.adobe.com/jnack/files/2012/07/CS6_hardware_recommendations.pdf
Refurbished 21.5-inch iMac 2.9GHz quad-core Intel Core i5
Originally released September 2013
21.5-inch (diagonal) LED-backlit display with IPS technology; 1920-by-1080 resolution
8GB memory
1TB hard drive
NVIDIA GeForce GT 750M graphics processor with 1GB of GDDR5 memory
Built-in FaceTime HD camera
$1,269.00
Save $230.00
15% off
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Refurbished 13.3-inch MacBook Air 1.3GHz dual-core Intel Core i5
Originally released June 2013
13.3-inch (diagonal) LED-backlit glossy widescreen display, 1440-by-900 resolution
4GB memory
512GB flash storage
720p FaceTime HD camera
Intel HD Graphics 5000
$1,229.00
Refurbished 13.3-inch MacBook Pro 2.4GHz Dual-core Intel i5 with Retina DisplayOriginally released October 2013 13.3-inch (diagonal) Retina display; 2560-by-1600 resolution at 227 pixels per inch 8GB of 1600MHz DDR3L SDRAM 256GB Flash Storage1 720p FaceTime HD camera Intel Iris Graphics | $1,189.00 Save $210.00 |