Used Tamron 70-300 LD F/4-5.6 or New Canon EF 50mm f/1.8 II Lens?

i have Canon Rebel XT/ EOS 350D wit d kit lens but now m thinkin of gettin another lens. my budget is tight therefore my choices for now are btwn these 2 lens.(d used Tamron lens cost half d new Canon).which one do u think is betta?i take mostly landscape, close up, potrait n group photos. really hope anyone can give opinion base on the photo quality, utility or anything dat matters. thanks a lot!!