As far as level one counts, Hi/lo and Hi Opt I don't counts 7's. KO, Uston adv. +/- and Silver Fox, among others, count the 7's as +1. I can't think of any level 2 counts that don't count the 7's. Counting the 7's is better for playing efficiency, but isn't necessary. Hi/Lo and Hi Opt I are both very good counts. Hi/Lo is the most popular system there is, it's basically the industry standard, and it doesn't count 7's. It depends on what kind of games you'll be going against. Assuming you're using a level one count, if you're playing shoes, you don't need a high playing efficiency. A high BC is what you're after. Against hand held games(single, double deck) is where the higher PE comes in useful. If you're gonna be using a level two count, then it's not an issue, becuase the system will be counting 7's regardless.
My first system was KO. I wanted to stay simple but add the better accuracy of a balanced count, so I decided on Hi/Lo. I'm of the opinion that "Simplicity is Power", so I stick with a level one count. I think simple and powerful is the best way to go. And for the vast majority of people, a level two system with an Ace side count, like Hi Opt II is not simple.