Did search for a mk1 thread first and couldn't find one, quick 'personal' comparison between the mk1 and mk2 first.
Please note this is my personal opinion only!
This is VERY basic test between Canon's 50mm 'Nifty Fifty' mk1 & mk2 mounted on the 7D and hand held.
mk1
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mk2
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The first things you notice between the 2 lenses are the mk1 is slightly heavier (both tiny so wont make any difference in the real world) also the mk1 is definitely better made, it has a metal mounting and a focal gauge where as the mk 2 has a plastic mount and no gauge.
The other main point is the focus ring on the mk1- it is part way along the barrel making it very easy to use but on the mk2 they have stuck it right on the very end and it's tiny so if you have anything other than toothpicks for fingers you may struggle with manual focusing on the mk2 - made worse if you have a hood fitted. NOTE: they are both fairly noisy when using auto focus with the mk1 winning out by a very small margin.
Right down to image comparison - to be perfectly honest with you, there is very little between the two, at least from the indoor / pop up flash tests I have done so far - I may amend this once I have ventured outside. If anything I think the mk1 is very slightly sharper 'on point' and has a slightly shallower depth of field than the mk2 - but you would have to be very critical in your shooting to really notice.
Excuse the dust on the shelf - the bikes are a nightmare to keep clean.
mk1 - 1/60, f/2.8, iso 125, aperture priority with pop up flash
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mk2 - 1/60, f/2.8, iso 125, aperture priority with pop up flash
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Cat - tricky as I can't replicate the 'pose' for obvious reasons
mk1 CROP - 1/60, f/2.8, iso 125, aperture priority with pop up flash
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mk2 CROP - 1/60, f/2.8, iso 125, aperture priority with pop up flash
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My conclusion - if you can find one and are willing to pay a little more for second hand then I would choose the mk1 - just because it is better built and a LOT easier to use in MF.
Whichever you choose you will be happy non the less - for the money both are pin sharp where they need to be and pretty quick. A very underestimated and unbelievably cheap little prime lens.
.DAVID.
1D Classic,1Ds, 400D. EOS-1n, EOS-5 Minolta Autocord,Yashica-A,Yashica-mat 66 Yashica-mat 124G ,Rolleicord IV & VB ,Mamiya C3,Mamiya C33, and a heap of other gear I keep collecting and collecting,Hi I am Steve and I am a photoholic 
