The "is" has a couple nice bits that the "i" won't have. A different front air dam, a spoiler, they typically have sport seats, and usually come in diamondschwarz, blue, red, and silver.
Here's mine, repainted (by PO) fire engine red, mudbogged somewhere, still straightening everything out.
I bought this "is" a year ago. I've only driven it home and for a week, when it ran like crap. The PO really did some damage to it, trying to fix it. It doesn't matter, he car is so much fun to drive, and, believe it or not, the faster I drove it, the better it got. This one has a lot of bugs, but the E28 is pretty hearty. 22xxxx miles is high, but the M30 is pretty tuff.
Do it. Spend the least amount now, because there WILL be maintenance needed. The good news is, the car lasts forever, and it's easy to work on.
p.s. the "is"s are a little more rare than the "i"s. Good luck!