Oh yes, Unity is a very powerful ring. We've tested it a lot, and I personally feel that it only makes sense if at least 3 people in group are wearing it. 2 people Unity didn't really work well, 3 people Unity is insane and makes everyone almost invincible (if all wearers have about the same toughness, of course). Can't imagine how it is with 4 people.
However, Unity takes up a ring slot, and many people are set on Ring or Royal Grandeur and Stone of Jordan. Therefore, having high mitigation just in case that not your entire group is wearing a Unity makes sense ;-)
98%, wow. I'm a monk too and currently at 93-94 with 10k Armor and 1k Allres (cannot get my specific resistance on a couple of items yet, might get it up to 1300) and 7% melee. What are your stats?
Exactly what Jamoose said. If you just enter some numbers in the calculator I wrote (yes, specifically for this purpose) it will show you that keeping numbers on par is better.
You can't put a number on it though. 75% doesn't mean anything. Go as high as you want. My monk is at 98% combined damage reduction, many wizards go for > 95% combined damage reduction, but there are also glass canons with less than 70% on each single damage reduction. Whatever works for you, but the higher the better because on Torment 6 every single percent reduction counts.
However, Unity takes up a ring slot, and many people are set on Ring or Royal Grandeur and Stone of Jordan. Therefore, having high mitigation just in case that not your entire group is wearing a Unity makes sense ;-)
Well, my monk is not a representative sample as she is a pure tank monk. You can see her stats here: http://www.diablofans.com/forums/diablo-iii-class-forums/monk-the-inner-sanctuary/84322-pure-tank-monk-build-gear?comment=23 (a bit outdated though, I've increased his toughness to 32m yesterday while lowering the health pool, so probably even more mitigation right now).
You can't put a number on it though. 75% doesn't mean anything. Go as high as you want. My monk is at 98% combined damage reduction, many wizards go for > 95% combined damage reduction, but there are also glass canons with less than 70% on each single damage reduction. Whatever works for you, but the higher the better because on Torment 6 every single percent reduction counts.