Unless you decide to go for TADS instead, I would suggest learning both I6 and I7.
Like C, I6 has a certain austere directness and simplicity, particularly if you have developed a programmer’s eye for reading code.
I7 has a more aesthetic beauty, both to read and to write, including the ability to concisely express surprisingly complex ideas in a single sentence.
It remains the case that certain things are easier to do in one language or the other, but nowadays I7 has the edge in these.
If you know both languages, since I7 compiles to I6 you can at certain points in your code drop from I7 to I6 for particular purposes, a bit like inserting in-line assembly code into C.