You can walk between those resorts over a bridge that takes you over the lake. It's not a bad walk but late at night its not as convenient as just walking back to your room if you are staying at Boardwalk Inn!
I think choosing the hotel depends on which theme you like best, since you have already settled on the Epcot area. Have you seen the pictures of the room interiors of the BC, YC or Boardwalk? I've stayed at all three each resort has its pros and cons, but its all subjective, of course. For me, the rooms at the Boardwalk are the prettiest in terms of decor. I also really liked the view of the courtyard we had from our very large balcony. The Boardwalk has the best theming (reminds me of Great Gatsby which I loved as a kid) in my opinion, and I loved that we had the quiet pool's jacuzzi to ourselves most nights. You can also get a pretty good view of Illuminations from this hotel, especially if you are staying concierge because you can see it right from the lounge area. My son loved the elephants around the Carrousel themed pool (with a big carnival slide) because he's a Dumbo fan (he's 5 now). There was a playground he liked, too. I think its a great resort for families or adults alike because adults have a lot to do on the Boardwalk as well, and there is a workout center. Some cons might be that the restaurants are pricey, some might find it inconvenient to have to leave the interior of the hotel to go to get something to eat (doesn't bother us, though), and this hotel has a busier feeling than say, the Grand Floridian. But I thinks its a great place.
The Beach club has an elegant seashore theme. Some think its less upscale than the neighboring Yacht club but I would say its just more relaxed...I think the Cape Cod-esque type decor is pretty elegant. We stayed here in concierge and had great service. It was one of our few trips were we received towel animals. Here we had great luck being alone in the quiet pool jacuzzi. We were disappointed by how early SAB closed (8pm, which was well before the time we'd normally visit the jacuzzi) and I didn't like the sometimes brutal splashing by very excited families at SAB. My son likes to swim, too but I ask that he not splash anyone! He LOVED the small pirate slide, though, and we had a very pleasant stay. We had a great time at Beaches and Cream and found the food surprising good (the ice cream and burgers were above average par). The Yacht club is has a nautical theme and the rooms are in blues, golds and off white. It had kind of a colonial feel to me, with lots of stars and emblems. I found both sides of the resorts maybe slightly less busy feeling than the Boardwalk, but not really anymore user friendly. It's still hard to get the refillable mugs filled at the YC, BC or BWI (all places you need to ask an attendant and there are only a couple places, plus there is no caffeine-free diet which is what I drink). Its easy at the GF, WL or AKL where you can refill mugs yourself. Not a biggie, though.
We stayed at the Boardwalk after a Yacht Club stay because we felt it was silly how much time we were spending taking the boat or walking over to the Boardwalk when we could have just as easily chosen that hotel (we tried the YC on our first Disney trip before we knew much about the hotels). So its not a big deal to get over there, but its certainly more convenient if you're staying right in the BWI
if you're going to be spending a lot of time at the Boardwalk area.