I'd say go with the 20mm. After a year of Having only a 50mm as my FL, I have decided on either 28mm/35mm as my walk around. But that is just me. :P But if you need something for lowlight, the 50/1.8 is nice, my favorite is the series E because it us a bit small.
A 20mm lens is really wide on 35mm film. But it will certainly add more variety to your available choices than a 50mm lens will.
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
Thanks for all the feedback guys, especially the Non-AI, AI, AIS thing made me checked more about the lens, its a AI unit which means the metering can and will work when paired with the lens, correct? the lens for now is up for sale at USD100, im trying to see if he is interested to let it go for around USD60, if yes then it will be a bargain, thanks guys!