mangaman74 said:
Given the choice, I would go with the blu-ray version. Ofter it will have a better picture/sound quality. With anime series there is also the fact that blu-rays take up less space - 13 episodes fit on 2 discs in 1 case, mid 20 episode series fit on 3 discs in 2 cases.
DVD is also 13 eps as 2 disks. You don't fit more episodes because it's a BD, just the same number in a bigger format.
UK BDs mostly are native HD (as manga don't want to release upscales), a large number of American and Japanese ones are just upscale tho'. If you have the DVDs* and don't care for lossless audio don't bother with them.
*that were actualy DVD quality, some FUNimation DVDs aren't, due to being authored baddly.