The most important question to consider when making your decision is ... what will you be watching on?
If you'll be watching on a computer ... the DVD is for you. Most computers and laptops do not have BluRay drives. A BluRay will not be watchable. So if you're primarily going to be watching on a computer ... get the DVD.
If you'll be watching on a HD TV, and you have BluRay player ... get the BluRay! But, you say, they are X $$ more ... what I'm here to tell you is ... they are X $$ better. There really is a huge difference in the quality of a BluRay, and you will enjoy watching the shows in the vivid, lifelike detail the BluRay offers.
If you don't have a BluRay player ... get one! They will be on sale cheap on "Black Friday" after Thanksgiving - I think I saw them for $29 from some retailers last year. You can probably even find a deal on-line and not have to get involved in the crowd insanity in person :-) Not only will you enjoy this show more, any movies or TV shows you rent (think Redbox) or buy are available on BluRay and again will give you a superior viewing experience.