Yeah I do agree that Kami is better in terms of exactly "whoops I'm out of power". That said, Gravi chest plate also recharges your tools. It's quite effective when mining.
Gravi is easier than Kami, but only because most people have already built machines, and AE, and a power subsystem when they start looking to get end-game armor. If you had researched down to Kami, but had no power or ME system, Kami would be easier. If withers spawned, Kami would be easier too. It's all about what you've already invested in.
As if you don't have to be COMPLETELY end game for Kami. You can get the whole quantum set for ~30 iridium ore. The only problem is the time consumed for all that uum to be created. Also, you need quite the power to feed the massfab. But still, if you're end game, you don't have what to do, you might as well invest your time and resources in to a Kami set.
Also, sorry for the double post, if you start looking in to Kami, you need nether and ender shards, well, if you don't have the best armor acquirable at this level of the game, you will get shit on by pigmen and Endermen. I remember what a hassle it was to get the stupid fire bat scanned without mid-late game armor in my SSP.
