About the "ring" system: Increasing level is increasing stats, the girl will be stronger. That's what you believe. But you there are few things you shouldn't ignore in this game: Heavy RNG base.
For example, you waifu is 150, her stat especially evasion and hit rate is increase. But you can see it? Let's see, in a normal sortie with 6 girls, 2 shelling phase. Your waifu will fire 8 times for 4 battles. Decide 50% hit and 50% miss and the increasing is 10%. Now the chance is 55% hit and 45% miss. Wow, within 8 battles i think you can't notice it ^^. It's even more worst with evasion since your waifu may not be targeted, so she can't "show" you her dodging pro :D.
So ring many or one, who will be ringed, is your choice. I think and i believe, it's the matter of love. Oh but if you are active long enough that most your girls reach 99, then ring all lvl 99 girls is good choice ( as long as you have money to buy ring :P ) because you will have motivation to play. ^^