Opinion seems quite divided on it, some people say use older OSes whereas others (such as me) always run the absolute latest-and-greatest OS that a machine can take. 9.1 will be slower than 7.5 as the extra features outweigh the more PowerPC-native nature of 9, but I feel the trade-off is worth it as it is so much more modern and 7 is not really a PPC OS. If you wanted to meet more in the middle between speed and modern-ness, 8.1 or 8.6 might be worth a look as they are lighter than 9 but have more PPC-native code and are still somewhat more modern than 7.x. This machine will never be a speed demon though (unless maybe if you install some serious upgrades like a G3 card), I have a 6100 with OS 9.1 and the OS itself is reasonably responsive but it takes literally minutes to boot and apps are slow on it - since getting my G3 I don't use it any more, it's too much of an unpleasant experience and there's no reason to use that rather than the G3, which I've been enjoying even more since getting a G4 upgrade for it. I haven't used anything other than 9.1 on it so I don't know how much slower 9 is than the older ones though. If you have a spare hard drive you could always try 9.1 on that and see what you think of it though