well, there are fans of barlows and there of the others... i am with the others. however that said barlowing a 6mm will give you more likeley than not more eyerelief than a 3mm. more glass to look thru too so it may degrade the image, or not depending i guess on the quality of the balrow and eyepieces in question.
anyhow, somone hwo likes barlows for visual work will come along and tell you the positives
i use my barlow/s for imaging