desktop or laptop?

i gotta big dilemma. i wanna have new machine soon. i'm not sure what to buy. desktop? i checked the prices of components and i was really shocked! i can have dirt cheap nice and fast computer with everything i want. laptop? it's small and practical and i've got everything by me wherever i am. second hand laptop? i bought old one in spring and i'm still using it, but everything better will be better. i don't wanna do snap decision. i think i'll buy new laptop, but it will be a little bit expensive. i will see. do you have some advices? tell me...