Welcome to the non-lurking part of canreef.
My tank is exactly the same size as your six footer. I would say, think about what fish you would eventually like to keep. If you think a small tang or two might be in your future than go with the six. The four footer will give you a broader choice of lighting options and the initial cost will likely be less.
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260g mixed reef, 105g sump, water blaster 7000 return, Bubble King SM 300 skimmer, Aqua Controller Jr, 4 radions, 3 Tunze 6055s,1 tunze 6065, 2 Vortech MP40s, Vortech MP20, Tunze ATO, GHL SA2 doser, 2 TLF reactors (1 carbon, 1 rowa).
http://www.canreef.com/vbulletin/showthread.php?t=50034 . Tank Video here
http://www.vimeo.com/2304609 and here
http://www.vimeo.com/16591694