I don’t really understand why people would ask about difference between two poems, as they are and should be different to qualify as two separate poems. I think you should rather ask about the similarities between the two poems.
 
In that case I would say, both the poems deliver a similar kind of message, inviting us to work. The poem I Believe presents the poet’s self-belief and the limitless possibilities in a man and inspires us to work in the present in order to achieve higher goals. Stopping by Woods on a Snowy Evening narrates how the poet-speaker wanted to enjoy the beauty of the snow-covered woods but he had to go, as he had other duties to perform. This gives us a message not to waste our precious time in the distractions and temptations of life, but to make a good use of this life by working hard, for this life is really short and we have many things to do, ‘a miles to go before we sleep’.