Transportation in Japan

What's the difference between ord & green car of JR pass

The green version lets you ride "green cars" (first class) in addition to ordinary cars.

The ordinary version lets you ride only ordinary cars.

In my humble opinion the green car surcharge is not worth it: the ordinary cars are amazing: spacious and comfortable.

Well, I wouldn't go as far to say that ordinary cars are amazing and spacious, especially if you have some luggage with you that doesn't fit in the overhead storage but that being said, stick with the ordinary cars, it's more than good enough!

I really depends how and when you are going to ride the trains.

If you are going to mostly ride the reserve seats on Shinkansen long distance high speed trains, ordinary will be just OK and not worth spending extra. However, if you are taking these trains in the rush hour morning and evening traffic, sometimes, there are no more seats, and you wish you had green car pass.
Some rapid trains in major cities have green cars where you have free seatings. These are shorter trains like the Rapid Airporter from Narita Airport (not NEX). Ordinary on these commuter trains mean you will be most likely standing for about 50 minutes from airport to Tokyo station.

On the Sanyo Shinkansen, west of Osaka, most Shinkansen trains have eliminated the green cars. These trains are called Hikari Railstars. If you are doing most of your traveling in west of Osaka, then I don't think you need Green pass.

I always get the "green" railpass because I used shorter commuter trains a lot and travel mostly around Tokyo and Chiba.