I'm up there quite a bit. If I were in your shoes (yeeehaw puns!), I would find a relatively fast-drying hiking boot with good ankle support instead of one of your choices. In a normal August it's pretty dry and most portages aren't too bad but that's nothing a good storm wouldn't fix. We all know what a little rain can do to a hiking trail, now picture the worst of those with a canoe over your head. There is always a chance that a simple 40 rod portage could turn into a dirty, painful, wet, and grueling experience after rain. Unless you're lucky enough to use a kevlar canoe up there, most canoes aren't even close to being lightweight or easy to maneuver on most trails so in my opinion, ankle support and fast drying are the two important pieces. That said, I have friends who only wear sandals. I think they're nuts, but they also make it up there a dozen times per season, versus the four or so trips that I'll make per season.
Around camp what you wear isn't quite as important. I'd go with a decent sandal, but let's be realistic... You'll probably go barefoot if you wade into the water or if you take a casual paddle on the lake, "light hiking" after a full and exhausting day of paddling and portaging is normally defined as a trip between the fire pit and the latrine, and not that weight isn't important in the BWCA but it doesn't seem like you are hardcore ounce counting this time so unless it's significant by your own standards, weight shouldn't make or break this one particular decision. Keep it simple, relatively light, and keep it comfortable! Trail runners, water shoes, sandals, high heels, or whatever you prefer to wear while enjoying breathtaking scenery. If you do choose sandals, I don't see anything wrong with Chacos (I'm assuming you aren't considering flip flops by any manufacturer), but in my opinion Teva makes a better product. I've done weekend hikes in mine without issue and they'll work fine if you do find yourself with a spare day, some extra energy, and a trail nearby.
Have fun! Also, do you know which lake you'll be starting on?