uhhh....one has a methoxy on the 3 position and the other has it on the 4? You could try reading any of the MANY posts and threads on both, they are radically different in effects, dosage, duration, availability, etc. Use the search engine.
Roughly speaking, in the roughest possible sense of the word rough: 3-meo-pcp is like K and 4-meo-pcp is like pcp. Not exact at all, but that's a general explanation.
^3-MeO-PCP is nothing like ketamine, and 4-MeO-PCP is kindof in between the two.
On topic, the most important things to know are that 3-meo is a lot more potent than 4-meo, and that 3-meo has a considerable amount of opioid activity, of which 4-meo is devoid. They're both fairly enjoyable, but have very different subjective effects.