Basically you want your slide on the opposite side of the "hallway" to the bathroom/bedroom. In our case, our hallway and the slide are both on the back (driver's) side of the trailer. In our neighbor's Fuzion, their "hallway" is on the front (passenger side) and the slide is on the back. Their living slide is on the front, so they also have better access to their kitchen than we do. In ours, which has the living slide on the back, if you have it in, you have to scoot over on the couch to get around the counter peninsula to get to the fridge, or you can go in via the garage but we usually keep the sliding door there locked while in transit. We just keep a cooler in the truck for drinks and snacks though so it's never a big deal. But again, we only vacation with it so your needs may be different for living in it.
My own personal pet peeve is slides that open up on the front. But a lot of 5th wheels have them, and some of the big ones with 3 slides really feel spacious inside because they have slides on both front and back. I just don't like them sticking out in front when we set up our nice "Patio" area with the awning, chairs, outdoor rug etc. We spend a lot of time sitting outside while camping, so it really just depends on what your needs/wants are in terms of what you'll be doing with it.
Can't help you on the 2 vs 3 axle. Ours is 2 and my husband was pretty adamant about only having a double axle, not sure why other than that's what his dad told him.... something to do with the middle tire wearing out faster than the outers or something.
