After trying the original one, the RT and the Grip versions, I have the following conclusions.
-They all write the same
-Seems like the blue ink on the original one is indeed lighter than in the new ones
-There are no color choices for the newer pens.
The original pen wins the prize for size and appearance, it has the dimensions of a regular pen, looks and feels like one
There is a tie between the Grip and the RT. I liked the thick grip of the RT, but after writing for a while that just doesn't really work, on the other hand, the Grip is too comfortable to be used during long periods of time. Ergonomics on the original pen are, sadly, terrible. Mostly due to the sharp edge where you normally hold it to write, hopefully Sanford will address this eventually
The original pen, its size means you can take it everywhere. The Grip takes the 2nd place because it is slightly bigger, and the RT takes the last place because it's huge.
One thing I noticed is that both versions with a rubber grip tend to pick up a lot of lint, it is an issue for me because they normally travel in a specific pocket in my pants, and they pick up a lot of lint, but the Grip one has at least some form of protection
The RT is very prone to leakage, because if the clicky thing is pressed accidentally when the pen is in a pocket, the tip might actually come out of the body and the ink is then transferred to the fabric (my pants are living proof of why you shouldn't carry the G-2 Mont Blanc in your pockets, and the Pen RT to some extent).
And the classic pen is available in 8 colors I believe, while the others are just available in blue, black and red.