we have stayed at the Poly and the Boardwalk Concierge, and there was no comparison. From what I have read and seen, the Poly Concierge is the best by far of all the resorts. The Poly C is in the Hawaii building, so if you stay in that building, it's a short walk right to the lounge. The lounge is huge...2 floors...the downstairs has all the food/drinks and there's an upstairs quiet area where you can relax, read and eat. The food is always excellent, but the best thing is the fact that they have drinks (water/soft drinks) out 24 hrs a day...my kids are already raving about how they can wake up at 2am and walk down to the the lounge and get a soda...(we're going in Aug)...while sitting in the lounge, you can look out the ceiling to floor windows over Bay Lake and out to Cindy's Castle....at night during the fireworks, they dim the lights in the lounge, pipe in the same music that is playing in the Magic Kingdom, and you can sit and enjoy the fireworks while enjoying a cordial or glass of wine...
the Boardwalk's concierge didn't even compare on any level...food wasn't nearly as good, no view to speak of, and you don't have 24 hour access...
As far as spliiting time, we do that almost every time we go...one of our fav things is experiencing the different resorts...we usually stay at an epcot resort and then switch to the poly concierge to enjoy the rest of the vacation in style...there is usually no hassle at all in switching...esp if you're staying concierge...you can just go to a theme park and have the concierge desk forward your luggage over to the new resort...
I have heard good things about the AK concierge...you should make a post describing it...
have fun!!!