Choosing a DAW is much like choosing a programming language & IDE in one, all your knowledge becomes domain specific, and as such, as soon as you get serious it's difficult to consider open source options seriously.
The only thing that is partly stuck to the program is the workflow. If you just know in which order to press the buttons and don't know what they do or why they do it, then sure, you knowledge applies only to that program. But if you know what the tools are doing then you can jump to pretty much any DAW on the market and make it work. You'll be slightly less efficient because of changes in workflow while getting used to the new workflow. And the bigger the difference, the weirder it is for the first couple of hours. Jumping from ProTools to Ableton Live is quite weird, for example.
I've jumped between ProTools, Ableton Live, and Reason with no problems. I've also played around with Logic, Reaper, and Cubase.
Reaper is quite nice, and works well. Also very easy to grok the workflow as it is a simple one that is quite like ProTools in that it relies heavily on an old workflow that would fit an audio engineer from the 1980s or 90s.
Reaper rocks for that price!