The chief advantage of an IF is that you usually get custom geometry. Since this is an old one, I assume that's not the case. Both will work fine and there's nothing wrong with eight speed. It's cheaper and at least as durable as nine speed. Two pounds of weight savings is nice, but if it's too small, it isn't worth it. I believe Rivs are measured center to top, so the 55cm IF is closer to a 56 Riv, but Riv top tube is going to be a lot longer. But that's partially compensated by the taller seat tube which gives you a smaller saddle to bars drop.

Only you can really tell if the size is right. See if you can put a few miles on both before you make up your mind.

The age is pretty much irrelevant.