Arctic... The cedar wood skirting will need yearly/bi-yearly maintenance (sanding and stain) to keep it looking presentable. You may want to do the math and see if it is better to cross the border and buy on the Canadian side. Your dollar is trading at about $1.25 over a CAD. 20,000 USD is about $26,500 Canadian so if you have a truck and trailer it might be worth a road trip.
100% wet test each spa before buying. There are many different way to create the swim flow of water and you might not like the way the water flows. Some use a propeller some use round jets some use elongated jets and you won't know without trying. Variable speeds allow use by different levels and sizes of swimmers. Separation between hot tub and swim area should be considered. Without separation do you swim in 104 degree water so you can enjoy the hot tub portion or do you keep the water at a lower temp so you can swim without over heating. Know what the warranty is and covers. Most warranty are pro rated over the warranty period. Service is very important to consider. Do they use in house service or 3rd party and what are the charges for them to come out for a service call. Living in Montana insulation is very important if you plan on using year round. If you go with the Arctic I would add extra Roxol rock wool to the open areas inside the spa leaving the equipment area enough room to breath. Can't comment on Endless as I have never seen one in my area. Good Luck with your purchase.
Most swim spa deliveries will require a crane to get it off the trailer and into place. Often having to go over the house and different size cranes cost different amounts so make sure you know those costs as well.
