Just like everything else. The newer one is better. Apple isn't Microsoft. They don't put a new sticker on their box and try to sell you the same thing for $200 bucks. The new one is way faster and has better range.
The thing is how much do you care about it being new? If you don't care look on ebay or craigslist. You can get the 4th gen one that works insanely well for $65-80 bucks or the new one thats $180 at stores for $100. I have the 5th gen in my apartment and the 4th gen at my parents house. I can't tell the difference tho we both have the Comcast bundle internet which is not insanely fast.
The speed on the router depends on your internet speed at home. Great you have a $200 router but if you have $15 DSL not going to make much of a difference.