There will always be something new and it will probably be available the day after you make a purchase. This happened to me on more than one occasion. I remember when I was getting ready to go a scuba diving trip to the Caribbean and put off purchasing a new GoPro camera for shooting underwater video. WOuldn't you know it, as soon as I got back from that trip I discovered my GoPro 2 had been replaced with the all new and improved GoPro 3. It happens. I've know people who put off making PC purchases for over a year, waiting on the newest processor from Intel and then get so upset when an even faster processor shows up a few months later... for less.
My advice to you, get the best you can afford NOW. Will there be something faster or more capable soon? Of course, there always will be. But if there is a computer or phone or device which can make your life or job easier, better or more productive right now, you shouldn't wait. The improvements are rarely worth waiting for and you could really be missing out by not utilizing available technology today. If I had waited to buy my underwater camera, I would have missed out on some amazing memories from those dives. I went on to use that same "outdated" camera to shoot an entire web-series on YouTube, about scuba diving in Honduras. Those videos look great (over 150,000 views) and the camera did just fine.