Which company has done ARM development right?

Neither, but Xilinx is vastly superior to Altera for embedded development. It's not even close. Unless you are developing data center servers, Intel does not care about your business. It's the reason they bought Altera and nothing else matters to them.

Your alternatives are:

1) Get a Zedboard and you can be up and running in short order on Xilinx using free tools to test it out. There is a vast support system, many examples, and you can just click a button to add most things to the BSP. I had a FreeRTOS+TCP app running in a few hours (although I had used Vivado previously).

2) Get and Altera SoC kit and spend months pulling out your hair trying to figure out what deranged maniac came up with the Altera tool flow. No examples, no support (look at these forums). The decision to use UBOOT and off the shelf DS5 seems to be the source of most of their problems, but I've been told they think their tools are wonderful. The Altera tools have been broken for many years (I first looked in 2014) and are actually getting worse, not better.

If you have a choice, RUN away from Altera SoC as fast as you can.