GEDmatch (on their Genesis site, going forward) allows uploading results from most of the major testing companies to be able to compare matches across the testing companies with others who have uploaded there. While this is beneficial, many testers at the major testing companies don't upload their results to GEDmatch. FTDNA provides the ability to upload results from other sites for free, though charges a fee to use some of the FTDNA tools. But my understanding is that newer test results from 23andMe are not compatible at FTDNA.
While I've only tested at FTDNA, my understanding is that if you test at AncestryDNA, you'll be able to compare to all the matches there, you'll be able to upload to GEDmatch/Genesis to compare to the matches there, and you'll have the option to upload your AncestryDNA results to FTDNA with the potential to compare to all the matches there. I also believe that AncestryDNA results are compatible with most of the DNAGedcom tools, though I have no experience working with AncestryDNA results (just FTDNA results) there.