which processors? haha intel has multiple types and so does AMD...
for all we know you could be talkin sempron/celeron, in which case id suggest sempron just because both are slow and sempron supports 64-bit.
athlon 64 and pentium 4, depends on what you're doing. athlon 64 is known for its gaming benchmarks, and pentium 4 is known for its hyperthreading and multitasking capabilities.
athlon 64 X2 vs. pentium D id go with athlon 64 X2 no matter what really..