Jeremy,
I'll have to weigh in with a vote for a northern entry, as well. I'm good with all the above reasons & have a couple additional. Re: bugs... what Mad_Mat said about the winds that time of year is right on. MANY of the camps I know of on Quetico, Cirrus, & Kasakokwog lakes are situated so as to allow adequate "breathing" with those winds. In other words, they are oriented so enough wind flow will minimize bug issues. Of course, when the wind fails to blow, you would be SOL anywhere.
Bear in mind that black flies hatch in streams & shallow moving water. The closer you camp to it, the more likely you are to have issues. Also, I'll second Prouboy's endorsement of "The Bug Shirt". I used it for the first time last year & loved it.
One of the very finest trips I've ever made into Quetico was 2-3 days after ice-out in 1996. Ice-out was really late that year; we went in during the last week of May. We encountered NO bugs of any sort (except spiders in a Cache River swamp). Frankly, though it got COLD at night, we got lucky as hell with weather. NO rain until the very moment I was loading my canoe back on the truck. BEWARE... that was one lucky roll of the dice! Nine out of ten trip reports I read re: camping in May feature remarks about cold nasty weather figuring prominently during significant portions of the trip.
One of the nice deals about the northern tier of Quetico lakes on the west side is that you have very FEW portages & those are generally really short. You can cover a lot of water while regaining your paddling fitness & gradually work your way into portaging shape. You'll also find all four species of game fish in those waters. You'll want to focus on the shallower bays, most likely, for fishing because the main bodies of those lakes are very deep & won't have warmed up much.
PJ makes good points about Pickerel Lake plus, if you're lucky, you get some of those nice sandy beaches, to boot. If you haven't been on the water since your early years, however, I'd be concerned about paddling in the tempests that can blow up on that lake. Yes, there are spots to get out of the wind BUT you just might be stuck in them for days at a time when the waves of big Pickerel Lake begin to roll.
Thar's my 2 cents....
Jimbo