If you’re going to be driving the HEDDphone, while the DAC FS version will do it, there won’t be a lot of headroom with quieter recordings. Worth trying, and if it works for your music and listening you’re done, and if not you can add an amplifier (one of the THX AAA models, a Jotunheim, a Lyr 3 etc.) and still come in around $1,500.
If you have much DSD content, that would also allow you to run the DAC FS in “Direct” (native) DSD mode - which requires an external amplifier for volume control (otherwise the DAC FS will convert DSD input to PCM so it can use its internal volume control and amplification).
The Pro FS, with a custom balanced cable, has ample power for the HEDDphone without need for an additional amplifier (though the above caveats apply for DSD replay). However, this is a +$2,000 unit, requiring a custom cable, that has a bunch of features you won’t be using, is much more complex to use, and really all it will get you over the DAC FS + amp is that it’s a one-box solution.
Using the Pro FS with an external amplifier is a bit pointless vs. the DAC FS with an external amplifier here, simply because they are going to sound identical if the amplifiers used are the same.